Is there a way to provide a pop up warning when creating a quote in Netsuite if the customer has past due AR?

You could use a client script deployed against quote/estimate with a

saveRecord

entry point where you run a search for past due open AR. Use a JS

confirm

if you want to let the user know and give the option to continue saving or cancel to return to the page without saving. You could also use a User Event script

beforeSubmit

entry point during

create

event which executes before committing to the database, run an AR search and throw an error if you want to fully prevent creation if any AR past due. That type of search will run relatively quick, but will impact UX. If you just want a warning and option for user to return to page or continue saving, I’d recommend the Client script. Client script could also run the search for past due AR after the entity is entered, and put a banner message at the top of the screen, which would have its own type of impact on UX too, but then you wouldn’t need a popup requiring the user to do anything. Definitely some options.