The rear seat belt is stuck in the retractor. It can’t be pulled out to fasten the seat belt. We’ve tried pulling gently and at different angles.
You need to replace it. Or, find someone willing to take it apart to find what is jamming it. But this can be difficult, since it is a failed safety item now. Most shops will want to replace it to avoid any liability.
Did you buy this car used? It might have been deliberately altered to be used on a child seat.
Call your Subaru dealer. Many/all? Subarus come with a lifetime seat belt warranty. See if yours is one.