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* returns value in the map and puts it in the currentMove.


return 1; // This doesn't look like the value from the map
..
return 2; // ditto
..
return existingMove = '...

Check where you placed this line:

else if (Validation(board) == false){
It is in a chain of if/else if statements.

Shouldn't it be clear of them. Always Test for a legal move after the user...

Do you check if the square is occupied before allowing a move there?
When the computer moves to a square that square should be marked as occupied.

Are you going to post your new problem? Or what happens now?