These windows were installed new in the late seventies.
The window weight (counter weight) strings are shabby and some have broken.
I guess the term ‘window weight’ is a doubtful term because the strings may attach to some form of spring-loaded device?
I see an access panel on the lower left side held in place by six wood screws.
Removing the screws gains nothing because I can not get the panel out due to other interlocking plastic parts and window position.
You'll need a replacement part from Andersen. When you get it , it will have the instructions how to do the job. I'd call an Andersen dealer near you so you can bring the information he will need to order the correct part. It's usually the numbers on the bottom corner of one of the sahes, but call them to make sure.
Hopefully, they will still sell the part you need. Sounds like your windows pre date the balance box in the top frame pocket.