When signing up for a free trial for BalletActive, a small pre-authorisation charge of $1.00 (around £0.73 in GBP) is placed on your card as a way to prevent fraudulent activity. The charge is credited back to your account with a few days.
Some card issuers may decline this hold even when you have sufficient funds in your bank account. If you find that you are unable to proceed with a free trial then speak to your card issuer for more support.