Why can't I send campaigns to a club membership tag?

You can only send campaigns to a Customer Tag. If you want to convert a club membership tag to a customer tag, you can do so using Queries.

Here is how:

Customer: is found in the following Customer conditions; 

Club Member > is equal to > Is An Active Member

Commerce7-Add-Customer-Query (1)

Once you Execute the Query, you can select Create Tag from List. This will be a Customer Tag, so you can send Campaigns to this list.

To learn more about building queries, click here