Time Machine not automatically deleting old backups
If you wish to delete your biggest backups (because Time Machine won't do it automatically), you can download TimeTracker, which will calculate the size of each backup, as well as show you which files are taking up space. From there, you can:

decide to delete all backups of a file (by using the Time Machine UI and selecting &quot;delete all backups of 'file'&quot;)
Add the large file(s) to System Preferences &gt; Time Machine &gt; Options &gt; Ignored files
Delete an entire single backup by opening Terminal, typing sudo tmutil delete  and dragging, from finder, the backup you wish to delete. It should say something like this in the end: sudo tmutil delete /Volumes/Backup\ Drive/Backups.backupdb/My\ Mac/2020-08-27-150021