When you move to standalone forms you have to play by their rules. Macro code does not always work as it did within Base. Not using switchboard or standalone forms, a brief reading showed how to overcome your problem. When using standalone forms the open
you are using doesn’t know where the document is located - you are outside of Base so to speak. From this post (click here) the normal opening of another form is with a specific setup of a push button. The post does mention another way - through URL.
I believe your wanting to open another form using a button doesn’t require any code. From the mentioned post:
Place a button to open the form somewhere, modify the Action on General TAB to open document/web page and select the document to open by clicking on the three little buttons next to URL.