Lightsail user config/connection confusion

Question

So I'm trying to sftp into my server(AWS lightsail, ubuntu) from my work machine (windows 10). The default ubuntu user is able to sftp and ssh in. However, the users I added aren't able to sftp or ssh in. I have tried to follow the solutions given here
The users are sudo users
The private key is in the right ppk format.
The public key is in the right place.
The correct ports are open
The authorized_keys file has the right permissions
I know this because last week myself and another remote dev were able to log into the server with one of the users. On putty I'm getting an invalid format error, so I redownloaded and reformatted the key, but I'm still getting an error. On Filezilla I am getting the error "No supported authentication methods". I want to be able to sftp as one of the users because they both have permissions to actual view files.

Edit: I don't mind if I can get the password login to work instead of logging in with the key. I just want to be able to move files.

Answer 1

So here was the solution after tearing out my hair: the format for the new public key wasn't right. The remote dev's key was correct which is why I thought mine was correct, but mine wasn't. So here are the steps I did.
SSH as root
cd into /home/user
nano .ssh/authorized_keys
paste the key in a new line. To paste in lightsail's terminal you paste into the clipboard, then right click on the terminal line.
Save and exit
Connect!