Time Machine Not Configured

I succesfully encrypted a hard disk on a time machine and I initiated to turn off a FileVault without knowing the risk of it. I didn't know it lead into decrypting the same hard disk so I thought it was best to remove the disk instead of waiting to finish it's decrypting. I checked it back and it says time machine not configured.

I tried to select a backup disk once again but it says just like image below

Here is what it shows on the FileVault recently

I did turned it off but it went back automatically on and I don't have any idea how to solve this problems.

I needed to backup data through a time machine in order to update the latest softwares cause it's been a years and I don't want to lose any data stored on it.

Thank you in advance for any helps.