Time machine wont back up all of a sudden....Error 402 and encyrpiton issue?

Hello,

A few days ago, I purged a lot of files from my 1 TB SSD hard drive and deleted them. My hard drive was getting full.

Since then, my 4 TB SSD external hard drive will not back up with time machine.

Time Machine initiates but It says my backup is not encrypted and asked me if I want to encyrpt my backup drive. It has never been encrypted.

I select close and I get

When I click details

Any ideas how to fix this?

Thanks!