Hello! If a candidate applies for resignation without completing 6 months in a company should we accept it or not?

You can't hold any employee, even if you don't accept it then he may chose to abscond. If he's following the protocol, then you shouldgo ahead and accept. It also depends on the policy of the organization. That's answering your query, other than that, if you want to hold him back, then do check with him the reason and see if that can be addressed and if you can retain him.

okay! but we have mentioned it that you can leave if you after 6 months but its only 4 months now

Then he would have to abide by that, then you need to consider his commitment to the company during such time. Will he be productive or destructive, is it better to let go of him as an exception or force him to stay back, all this is situational.

okay! let's see what the employee says and what problems facing by him.