In order for you to generate invoices for upcoming billing, your members must have a scheduled billing assigned on their Billing tab. You can use the Membership Report to find a list of those members who do not have a schedule so you can make the necessary adjustments. In your report, filter on Active membership, and add and Additional Criteria/Filter for Total Recurring Fees = 0. The results, if any, will indicate that you have active members that presumably should have a fee schedule but do not.
See Membership Report for further details on generating this report.
If you have active members who do not have scheduled billing, and are expected to, you will need to update the scheduled billing associated to the membership, on the member's Profile tab. This will require an upgrade or downgrade. Perhaps, for example, when the member joined, accepted them at no charge but now need to update their billing. This would require a upgrade or downgrade.