Bob wrote:

I think its iron and I don't have access to the full T most of it is under
the concrete floor. It could be the threads are just rusty and corroded. I
am going to try and clean them out with a wire brush. If that does not work
I don't know what I am going to do.

Call a plumber?

I still can't tell for sure, but it sounds like you're talking about a
drain cleanout access?

If the threads have been destroyed on the fitting and you don't want to
replace it at present, one possible makeshift solution would be to fit a
4" to 3" reducer into the present opening, using an epoxy for sealant to
account for the failed threads. Then plug the 3" opening. Still have
access for cleanout, albeit somewhat restricted, and should be able to
get it to hold water.