Incomplete subscriptions happen when your payment method method could not be used to pay for the initial subscription payment.
While your subscription is incomplete, you will remain on the PAYG package. You then have 23 hours to activate an incomplete subscription before it expires. In this timeframe, you will able to activate your subscription by retrying the failed payment. This can be done by following the instructions on the screen right after the payment fails or trying to start a charging session. But you will not be able to start a different subscription while having an incomplete one.