Is your internal HD or ssd nearly full?
About Time Machine local snapshots - Apple Support You can read here:
“Your Mac counts the space used by snapshots as available storage. Even so, Time Machine stores snapshots only on disks that have plenty of free space, and it automatically deletes snapshots as they age or as space is needed for other things.”
If the internal drive is nearly full, create some more space by deleting items or moving large items to another external drive. TimeMachine backups can include external drives. You can e.g. move your photolibrary to an external drive Move your Photos library to save space on your Mac - Apple Support
Maybe you find this link interesting Free up storage space on your Mac - Apple Support