Ok I never post just lurk but due to financial constraints need some advice.

I have a toilet that the cut off valve will not function correctly. I have replaced all the guts in the tank but any water that is suppose to stay in the tank flows to the toilet (and due to another issue the toilet is clogged cannot turn on my main water without risk of quickly making a horrible mess.

Is this piece called a “wax seal?”

I have 2 other bathrooms I just need to be able to turn main water on and toilet fill tank and stop per usual as I can use others until future corrections. Any advice is welcome thank you in advance.