Just make sure the field "Weeknumber" is spelled exactly the same in both tables. That should do it.
Keep in mind that Qlik is Case Sensitive!
The one and only way to connect tables is by naming the fields correctly. Qlik has the opinion that if something is the same thing, you should give it the same name. If something is not the same thing, it should not have the same name.
I know, sounds pretty logical, but apparently not every software company thought of it that way the last 50 years .