Strangely enough, this is not a good question - we cannot work out from that little what you are trying to do.
Remember that we can't see your screen, access your HDD, or read your mind - we only get exactly what you type to work with.
So when your code throws an error and we have no access to yoru code, how do your expect us to be able to tell you how to solve it? That's like asking me to proofread your resume, but refusing to let me have a copy incase I see something you don't want me to know ...
So all we can do is talk generally.
Start by looking at the error message:
Warning: mysqli_error() expects parameter 1 to be mysqli, null given in C:\wamp64\www\PhpProject3\login_submit.php on line 6
This tells you a lot of information:
1) What problem it found. In this case you are calling a function that expects at least one parameter, and that parameter can't be null.
2) It's in your file "C:\wamp64\www\PhpProject3\login_submit.php"
3) It's line 6 that gives the problem.
So open that file in the editor, and look at line 6. What are you calling? What are you passing it? Where does that parameter get given a value?
Most probably, you are using a variable that you haven't assigned a value yet - by since we can't see any of your code, we can't tell...