I had a problem with my luks encripted external drive and it war driving me mad. I *suspect* the issue is caused by some poor handling on POP_OS/GNOME/something-in-between file GUI interface thingy (I do not understand the linux software ecosystem enough to bother to find out and report the bug in the appropriate channel). Just posting this link here because it was the only one that saved me, and it is well written so I can understand what I was doing:
https://trendoceans.com/mount-luks-encrypted-drive-partition-in-linux/