Congrats on deciding to overcome the challenges associated with your condition!
Your condition, while rare, is not an automatic no-GM situation. Here is some advice for successfully world-building for your RP scenarios and campaign despite lacking the "visual mind's eye" (please note, a lot of these will take effort):
Collect (or Create) visual resources
1) Search engines are your friend
Use numerous image search methods with varying keywords related to what you are trying to go display. Need to describe a castle? Find images or videos of castles. The same can be done with characters, items, costumes, jewelry, and so forth.
Unsplash.com, Pexels.com, and Pixabay.com are excellent resources for various images - people, landscapes, fantasy, structures, and more... best of all all the images are CC0 images or otherwise royalty free and license free: as in free for both personal and commercial use (certain minor limitations apply to commercial use - mostly don't claim it's yours or make compilations of other's works, check licenses on each site for details - but shouldn't affect RP stuff in any way)
2) Fiverr (or similar) is also your friend, if you are willing to spend a bit
If you have a bit of cash, then you can ask artists to create what you are looking for, or modify something you found that is 'almost'. When asking others to modify a pre-existing thing, however, be aware of copyright, license, and usage restrictions.
3) Photoshop or GIMP is your also your friend
If you don't, or can't, pay someone else to do it, give your own artist's chops a try. Even when lacking a visual mind's eye, with the undo features of modern graphics software, you can alter (and mess up) to your heart's content, and still undo anything you don't like how it turned out. Just remember to save often. As a tip, save alterations under different names, so that you can go back to a specific point in time - takes more file space, but allows you more latitude in your work. GIMP, by the way, is free.
4) Bryce, Daz3D, Blender are also your friends
Daz and Blender and are free 3D software which are quite capable in their own respects. Strong communities with lots of tips, tricks, and advice support both; Blender even has a Stack Exchange site. Bryce is not free, but is usually worth the price tag - and is still considered one of the leading landscape creation tools. In conjunction with one of the other two (it already is configured to work directly with Daz) it can support buildings and characters as well. Use existing resources (lots of free stuff out there online) and build your scenes (towns, buildings, maps, you name it), or even learn to make your own resources.
5) Use Props
Always fun to have the real thing (or close approximation) handy for reference. Nothing says light saber like some red and blue giant pool noodles, duct tape, and electrical tape. Plus you get to bash each other after (or during) game for fun (and profit?).
On a more serious note, a yellowed piece of paper with the note the characters just found, a Chinese puzzle box with something rattling inside of it for the players to actually solve, a cracked chess piece with what looks like a blood stain (a bit of paint or even certain types of markers work), can do wonders to set a scene and tone to the game.
These visual collections can be used to 'show' what you want, can be composited to 'create' what you want to see and/or show, and is all around useful for both you and your players. When you find a picture that you like, decide it is a picture of the Plain of (random name) and just behind that out-thrust rock is the City of (another random name). You can show it to the players if appropriate, or simply use it for your own personal reference to aid your descriptive verbiage.
Collect (or Create) aural resources
For those with a visual mind's eye, a bit of background sound will help them build a scene. If your locations and/or characters have their own theme music, or associated background sounds, a bit (or a lot) of work with some music software or websites where musicians post their work freely for everyone to listen to (or spend some resources to collect music files and/or sound clips) and you can have your own SFX library at your command.
Collect Descriptions
If you find that your aphantasia extends to preventing you from coming up with verbal descriptions of visual effects, then there are still a couple of things you can do. You can read the works of others, and collect quotes. Find bits of verse and descriptive text, make a copy and keep a portfolio of useful descriptive bits. When planning out adventures and locations, base descriptions on certain of your snippets. To keep your descriptions consistent (and unique to each location), keep notes so that you don't overuse any particular snippets.
Also, walk around (or use Google Maps Street View for distant locations) and verbally describe to yourself what you are seeing. Write your descriptions down. Touch things (appropriately, mind you), smell things (again...), take pictures (yet again...) and listen; in other words gather sensory data, and practice putting it into words. Describing the smell of a place as rank, musty, but with just a touch of the lingering memory of fresh bread, and your non-aphantasia players will build the image without any further help from you.... Plus, it's a great excuse to get out and see the sights, visit museums, and other places of local color.
If you can record the sounds and images of your walkabouts, this can help with your later descriptions as you (probably?) can't pull up the visual memories of your experiences.
I hope this advice is helpful.
