Dynamic Forms has two versions.  A Public version and a DMACC version.  The DMACC version 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 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.  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 that sent you the form link to obtain the correct URL prefaced with the single sign on URL above.  It's possible the form is meant to be public, which means you'll have to choose the Create New Account link and and generate a password to login.