Ok I can quickly tell the big issue here.
You shouldn’t and can’t use DAX functions inside the query editor. You should be using these inside of the front end of Power BI.
That area only accepts Power Query functions which is a whole different beast.
Definitely start working through all the content at Enterprise DNA Online here. It seems there is some initial fundamental concepts that you need to get your head around.
Start here
Then you’ve got plenty of things to move to from here.
Check out this post here around some recommended orders for courses.