Can I apply if I have not yet received my first international client payment?

A bank account with no international receipts in the last three months does not automatically prevent signup. You can submit a signed contract or other business evidence for manual review instead of a statement showing recent international receipts.

If you have a genuine overseas client contract, follow that alternative-document route. If you have neither a contract nor other evidence of international business, ask support what would establish your business activity before expecting approval. Do not create an invoice for work that never happened. The alternative route is a way to submit evidence, not a promise that an application with no business evidence will be accepted.