Time Machine not deleting old backups on Mac

If the interface isn't working, you can force-delete backups via Terminal:

Open Terminal (Cmd + Space, type "Terminal").

Type tmutil listbackups and press Enter to see all backups.

To delete a specific backup, use this command (replacing the path with the one from the list):

sudo tmutil delete /Volumes/[DriveName]/Backups.backupdb/[MacName]/[YYYY-MM-DD-HHMMSS]

(Note: You will need to type your Mac password; it won't show characters as you type.)

Or

Sometimes the space isn't full on your external drive, but rather your internal drive is full of "local snapshots" that haven't synced yet.

In Terminal, type: tmutil listlocalsnapshots /

If you see a long list, you can thin them out by typing:

tmutil deletelocalsnapshots / (This will clear the local cache and often fixes "Disk Full" errors).

or use the Disk Utility in the Utilities folder to delete Snapshots.

If all else fails

If your backup database is corrupted, Time Machine may stop deleting old files entirely. If you don't mind losing your history and starting fresh

Open Disk Utility.

Select your backup drive in the sidebar.

Click Erase and format it as APFS (for modern macOS) or Mac OS Extended (Journaled) for older versions.

Go back to Time Machine settings and select the "new" clean drive.