Is there any liquid to stop leakage inside pipe or fitting
The honest answer is no.

There are things that will fix your problem, but they will cause more harm at the end.

It is like a pill that will only fix the problem, but have no side effects.

Your list of wants is long:

You want something that is liquid inside the pipe, find the leak inside of the pipe, and then dry inside of the pipe.

You are also looking for something that will attach and block to the pipe at the hole, but not attach to rough spots or connectors in the pipe.

It will also not harden around whatever is at the end of the pipe.

There is nothing that will only do what you want.

Your list of fixes is short:

The solution is to either put a smaller piece of pipe in the one you think leaks, and use that instead, or trace the entire pipe to find the leak.