Time Machine won’ t delete some older backups because doing so would lose access to the files within them. It automatically “ thins” or deletes backups so that it retains hourly backups for the last 24 hours, daily backups for the last month, and weekly backups to the first backup you created in that series.
Although you may be able to delete old backups manually, you’d be wisest not to, as that would remove files from your backup history that you’d be unable to replace. If Time Machine has been backing up to an APFS volume, then those backups are stored as snapshots, which can’t be moved or copied to another volume or disk.
To resume making backups, you have a choice of changing the drive that Time Machine backs up to for a fresh one, or erasing your existing backups and starting a new backup series. One way to avoid this problem is to back up to two disks, starting the second when the first is half full. Time Machine will back up alternately to them, and together they’ll give you continuity so you can erase one when the other is still going strong.