When your Time Machine backup is full, there are several steps you can take to free up space. First, use the Finder to check what files are taking up the most space in your Time Machine backup. To do this, open Finder and select 'Go' then 'Back Up'. This will show all of the files that have been backed up by Time Machine. You can then delete any large or unneeded files from this list to clear some space.
Another option is to purchase an external hard drive with a larger capacity than your current one. You can then use Disk Utility on MacOS to move your existing Time Machine backup onto the new drive in order to increase storage capacity.
Finally, you may also want to adjust how often Time Machine backs up your data. By default, it will back up every hour but you can reduce this frequency if necessary. Go into System Preferences > Time Machine and change ‘Backup frequency’ as needed for increased storage capacity on your existing drive.
Taking these steps should help free up enough space so that you continue using Time Machine for backing up important data without further issues.