CL210 - Adding compute failing due to no valid host was found

Currently I am following the CL210 Online course.

In chapter 6, section 4 I have to add a compute node to the overcloud. I am following the exact steps (copy & paste), but the deploy is failing with:

NoValidHost: No valid host was found. There are not enough hosts available.

My exam (EX210) is planned for Friday 9th of Feb.

I need this to have this sorted out quickly. Thanks!