Hi @snips11,
Welcome to the forums and thanks for creating this post!
The form (salary-guide) found that exists on your static HTML page is on salary-guide.html. The page (become-a-member) that the form (members) is on (the form not found by Netlify) is rendered by a NextJS function, because of this, the HTML version of the form doesn’t exist while building. You’ll need to add the form as a static HTML version on the website. Once the form is detected by Netlify, it doesn’t matter how the form is submitted, and it should work for the dynamically rendered form.
This is mentioned under [Support Guide] Form problems, form debugging, 404 when submitting for If you’re using a pure javascript form instead, this additional step is required.
Hope this helps!