Related Questions:
Why isn't the Sync button showing on my Invoice?
Why can't I Sync an Invoice to Mews?
Why won't my Invoice Sync?
Answer:
Prepayment Invoices need to be Paid in the full amount before Syncing to Mews. When a finalized Prepayment Invoice is paid in full (the Amount Due is $0) the Invoice will automatically be synced with Mews.