Installing a Bidet and now the coupling (connector) leaks. I may have damaged the connecting plastic pipe
It looks like you have "cross threaded" the plastic pipe. (The term comes from the new set of threads you have accidentally cut into the pipe, which cross the original threads at several points.) 

The only repair that will work here is to force the metal nut straight onto the end of the pipe, forcing it to re-cut the original, correct threads. This will take some patience, some fussing, and some force, because the nut will want to follow the new threads, and you must keep it straight. 

If this was my problem, I'd remove both the fill valve and the T adapter, and use a sharp knife to adjust the threads in the plastic fill valve. Then I would clamp the adapter metal nut in my vise and assemble the two pieces while working in a more comfortable position than under a toilet. Once the nut is screwed straight onto the plastic, it will be much easier to assemble correctly next time. 

Then take it apart and carefully re-install everything. 

If you can get the fitting assembled straight it should not leak as the seal is provided by the ends of the T adapter and the plastic pipe -- the threads serve only to lock everything together and do not hold back any water. 

The plastic threaded part that you've damaged is an integral part of the fill valve assembly. If you can't patch the leak in place, you will have to replace the entire fill valve.