I'm trying to password protect a USB drive and get an error

GTR81 wrote:

That worked! I tried it again this morning and it worked. I guess I just try in the morning as opposed to the end of the day, when I have been using the computer all day.

You don't necessarily need to wait that long. I had this occur the other day....got distracted & tried again about 5-10 minutes later and the drive was able to be erased/unmounted. Other times it may take a much longer wait.

This issue has been around for years. It is so unfortunate Apple prefers to add more new features before fixing bugs in old features.

Make sure to select GUID Partition Map when you erase the flash drive. Your screenshot appears to indicate the drive is set to Apple Partition Map, which is not compatible with APFS format.

And to password protect the drive you should use APFS Encrypted format.

See this for more information -> Encrypt and protect a storage device with a password in Disk Utility on Mac - Apple Support

The error mentions not being able to unmount the volume(s) on that drive. Wait a bit and try again. Many times Spotlight Indexing is the reason.