How can I resolve path problems in PHP

Look at the error message, it's pretty clear and explicit:

Warning: include(..admin/config/dbcon.php): Failed to open stream: No such file or directory in E:\xampp\htdocs\rajpie-ecom-one\functions\myfunction.php on line 4

It's saying that line 4 of your file "dbcon.php" is trying to include a specific .PHP file at a very specific location:

E:\xampp\htdocs\rajpie-ecom-one\functions\myfunction.php

And it's saying that some art of that path does not exist, or isn't available at that moment (for example a networked drive that is located on a computer that isn't switched on).

There is nothing we can do to help you: we have no access to your computer or it's file system!
So start by looking for the file using Windows explorer: follow the path explicitly and see if the whole path is spelled correctly, and that the file does exist. If it doesn't ... well, that's obvious.

If it does, start checking access permissions for the user that is executing your code.

Sorry, but we can't do any of that for you!