Hi Paula

That is a strange message - even if you had another course as a completion condition of the course you should still be able to access the course you just wouldn't be able to complete the course.

Is this course part of a program and it is in a course set that requires an earlier course set to be completed?

Do you have any third party plugins - under Site admin > plugins > additional plugins around enrolment, visibility?

if this doesn't help are you able to create a support ticket through your partner with what version number and enrolment methods you have used ?

regards