There could be a few reasons:

1. Your lftp version is lower than 4.3.0, see Changes.
2. You're using Debian stable version.

According to this answer:

The Debian stable version might not have the attach feature though, but you can still list the active lftp processes with pgrep.

But you can trick the shell if you set cmd:move-background-detach setting to false and use the fg command to bring lftp back. See man page.

There could be a few reasons:

1. Your lftp version is lower than 4.3.0, see Changes.
2. You're using Debian stable version.

According to this answer:

The Debian stable version might not have the attach feature though, but you can still list the active lftp processes with pgrep.

But you can trick the shell if you set cmd:move-background-detach setting to false and use the fg command to bring lftp back. See man page.