Over time it is possible for this issue to occur as Time Machine may not be able to purge files ... especially when there are a number of very large files that need to be backed up frequently. Typically, this can happen with VM "machines."
Regardless, the next step, assuming that these backups are critical to you, is to first archive the existing data on the the TC's internal drive to one connected to the TC's USB port. Obviously, you would need a large enough USB drive for this process.
Once properly archived, you can then erase the TC's internal drive and, when complete, start a new Time Machine backup set.