The reason you can’t select a Class or a batch can be one of two things: 
1. That specific distance or batch you are selecting is full or sold out.

2. You are not fulfilling the required Batch/Class criteria, this means that you are trying to enter a Class or batch that has specific rules for entering. This can be a specific age category or team criteria. Some Classes require you to be entered into a team of let's say 4 members and you will not be able to enter the class until you are part of a team that consists of 4 members. 

Click video for more info on class selection. 

In this case, go back to the previous page and add a team with the specified number of members in it.