Error: “Credits cannot be applied on the selected rate at this time.”
What this means
This is always a rate configuration issue.
The rate selected for that day of week does not allow credits.
How to fix it
Go to Rates
Select the appropriate day of week (Mon/Tues/Wed/etc.)
Edit the rate in question
Make sure Allow Credits = YES
Save
If credits are not enabled on the rate, they cannot apply at checkout.