I bought a bidet sprayer handheld to install on one of my toilets. I followed all the steps given and it is leaking where the water supply line and T adapter connect. I tried to remove the bidet and rehook the toilet up as is and now its leaking at supply like to bottom of toilet. I know the supply line is old but its one that can't be replace. I have tried adding a washer and plumbers tape nothing is working.
Probably what happened is the black plastic nut on the supply line cracked. It looks pretty old and might have been over-tightened when you hooked it up to the bidet. You said the supply line can't be replaced - but it can. The supply line looks like pex and runs straight out of the wall. Is there a shutoff for it? Or did you have to turn off the water at the main?
I don't know how handy you are but if it's pex or something similar and there's no shutoff you should install one at the wall. You can get a 1/4 turn shutoff with a pex connection at a home center store. Purchase a new flexible supply line for a toilet so it will have the correct size fitting to attach to either the tank or the bidet.