We've experienced this problem, even with the Payment Gateway properly linked.
A great deal of students have had issues where the "Proceed to Payment" button has been unresponsive - even without any errors in the form validation / accepting terms & conditions.
Normally, I've been able to assist them by requesting the use Google Chrome instead of IE / Edge - however Edge is technically still supported by Canvas / Catalog so I am unsure why it would react this way.
During InsCon '19 I met with a Catalog Engineer & he was not able to think of any obvious reason this was happening - we are considering adding a notice on the New Enrollment page to suggest users only use Chrome.
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