Time Machine disk not showing up

I want to do a single TM Backup on a Mac before upgrading it. I created a partition on an external volume for this purpose. However, when it comes to selecting the backup disk, the partition is not showing up. All the partitions on the volume are formatted for MacOS Extended (Journaled).

What is the problem here? The amount of data on the Mac that is to be backed up is small. The partition is set to 50GB because it's only one snapshot.

Is there a minimum limit for backup disks?

I need to be using this partition tomorrow. Any help appreciated. Thanks.