Time Machine QNAP

That's the old TM system. You may have to erase the drive and reformat APFS - case sensitive in order for it to be used by TM for backups. You won't be able to use it for anything but TM.

Make sure your drive meets these requirements which are more stringent with Monterey: Types of disks you can use with Time Machine on Mac

With earlier Time Machines trying to delete a sparse bundle or a file would lock up the Trash bin and force the user to erase and reformat the drive so you might just as well erase and reformat the drive as APFS (Case-Sensitive).

Note: I don't use NAS so have no first hand experience with TM on a NAS.

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

Have you been using the QNAP currently for Time Machine?

What do you want to erase it?

What Mac model, year and system are you running?

Post a screenshot of what you're seeing in the TM drive. Since Monetary it looks like this:

QNAP FAQ

https://www.qnap.com/en/how-to/faq/article/how-to-delete-time-machine-backup-image-to-free-up-space#:~:text=To%20delete%20time%20machine%20images,image%20and%20click%20%22Delete%22.

Use Disk Utility to erase the drive and reformat it.