Dynamic Forms has two faces. A Public face and a DMACC face. The DMACC face uses your DMACC credentials and has a single sign-on URL prefacing the URL to the form. This single sign on URL will often get left out and you'll end up at the Public login which won't work with your DMACC credentials.
To know for sure if you're at a Public (wrong) login is if the login screen gives the option to Create an Account, or if you click Forgot Password and it doesn't take you to a myDMACC reset password page, but tells you it will email you.
One thing you can try is to just log into the Single Sign on URL and locate your form via My Forms once logged in. Or log into the Single Sign on URL and after you're logged in, try clicking the URL again that didn't work previously. Because you're already authenticated you may get access to the form.
If neither of those options work, you'll need to work with the department you received the linked URL from to obtain the correct URL prefaced with the single sign on URL above.