Problems connecting my database up

When you get an error message, read it carefully - it contains a lot of information that can help you!

Fatal error: Uncaught mysqli_sql_exception: Unknown column 'lpa_client_username' in 'where clause' in D:\xampp\htdocs\LPA\websystem\LPA\login.php:63

There is a lot of info there:

Fatal error: Uncaught mysqli_sql_exception: 

This can be ignored - it's telling you that it's a fatal error and it can't continue after meeting it instead of a warning which it probably can continue with.

Unknown column 'lpa_client_username' in 'where clause' in D:\xampp\htdocs\LPA\websystem\LPA\login.php:63

There is lots here:

Unknown column

What the problem was

'lpa_client_username'

What column it can't find

in 'where clause'

What part of the the SQL statement was it referred to in

in D:\xampp\htdocs\LPA\websystem\LPA\login.php:

Source code file it found the problem in

63

Line in the file.
The rest of the message you can ignore for a simple problem like this.

So you need to do two things:
1) Go to line 63 of login.php, and find out exactly what SQL command you sent - that'll probably be set on that line, or a few lines above it. What column(s) does your SQL command WHERE clause use? What table(s) does it reference?
2) Go to your database and look at the table definition: what columns does it contain?

Probably, you mistyped the column name, either in your PHP code, or in your database!

We can't fix that for you: we have no access to yoru code, or your data!

This is a syntax error: you should expect to get them every day, probably many times a day while you are coding - we all do regardless of how much experience we have! Sometimes, we misspell a variable, or a keyword; sometimes we forget to close a string or a code block. Sometimes the cat walks over your keyboard and types something really weird. Sometimes we just forget how many parameters a method call needs.

We all make mistakes.

And because we all do it, we all have to fix syntax errors - and it's a lot quicker to learn how and fix them yourself than to wait for someone else to fix them for you! So invest a little time in learning how to read error messages, and how to interpret your code as written in the light of what the compiler is telling you is wrong - it really is trying to be helpful!

So read this: How to Write Code to Solve a Problem, A Beginner's Guide Part 2: Syntax Errors[^] - it should help you next time you get a similar problem!