There are a couple of different reasons why your customer can't book a class with an active series. Here are the factors you should check for that customer:

Check to make sure that the Series they are holding is under the same Series Type that is allowed by the class in the Scheduler. If the scheduler for that class isn't selected for this specific Series Type, this is why the customer cannot book into the class.

If the Series or Contract are set to 'Yes' for the Activation Option 'Activated on First Class Booked' it's possible the Series/Contract has an activation date outside the class date. To check this, locate the customer in the booker, then select series. You will locate the series or contract and find the activation date. If the date is past the date of the class in question, you can click to edit this and update the activation date to encompass the class date. (please note: updating the activation date is a permission-based setting, if you cannot do this please request your admin to)

If you create your scheduler with a specific class credit requirement, it's possible the series attempting to be used to book a class doesn't have enough credits. Please note, you cannot use multiple series in one class booking. The customer will need to purchase a series or contract with the right number of credits, or the scheduler will need to be updated to reflect 1 credit.