finding 1 unknown

Do not worry about the last error in your sheet, but fix the first one and then the next, etc.

You can't use a variable unless you have defined it. And Mathcad reads from top down and from left to right. You use a variable which is defined in the next line - this does not work.

From what I see in your picture: Don't define cos(theta):=1. This definition does redefine the cos-function to return 1 for any argument given which sure is not what you intend to do. Rather set theta:=pi/4 to get what you want.