The error got nothing to do with the shop scene, it is from a wrong message code you use in a message box.
I guess, you show something in a message box before opening the shop scene, and that's when the error happens.
It is most probably a non-existent weapon icon/name that you wanted to show in the message box or another incorrectly formatted message code.
Or it can be a message code used in the description for one of your weapons. That text is the only one which uses the draw_text_ex method for drawing the text, and the error comes from that method.
If you edited the shop scene or got a custom one from a custom script, we will need to see these edits/script to know what causes the error (if none of the above did).