How To Fix SyntaxError: JSON.parse: unexpected character at line 2 column 1 of the JSON data

I'am New Here,i'am starting to learn some HTML,PHP and other code,then i try to build something with it,My question is why i always getting this : "SyntaxError: JSON.parse: unexpected character at line 2 column 1 of the JSON data" when i try to login in my PHP script,I attached my script with the SQL DB.Sorry for my bad english. Script