1) It is not clear if it is the iMac's drive that is full...? Or if it is the Time Machine backup drive that is full...?
If your iMac's drive is full, then the system does not have room to update the Time Machine backups.
If your Time Machine backup drive is to small or full, then you need to erase it and start over.
2) You should never move Time Machine backups to the trash, because now they are stuck in the iMac's Trash.
At this point you need to eject and disconnect the Time Machine drive from your iMac an set it aside.
Then check the iMac for Free Disk Space as per > Free up storage space on Mac - Apple Support
3) If there are Time Machine backups in the iMac's Trash, then the backup drive must be erased to free up Trash.
You can use Disk Utility to Erase the existing Time Machine backup and start new backups.
Or optionally replace the Time Machine backup drive with a larger external drive.