I can't open the first Unit in my Course

I have been testing the use of forms for the Course I have developed. To do this I added new Units to my course and moved them to be the first two Units of my course. After my testing I no longer needed the two new Units so I deleted them from the Course

Since then, when students log in to the Course the first unit is 'grey-out' and below the title there's a message 'Previous unit must be completed successfully'

What's the fix??