Why can't I mark an invoice sent?
Check its draft status, positive total, action permission and posting date.
The separate send action needs Send invoices permission. The invoice must be a draft with at least one line and a positive total. Posting uses the invoice date, so a closed or locked period or the backdating limit can block it. Nonprofit also applies its posting setup and coding checks. If creation saved a draft before posting failed, open that draft instead of creating another invoice. See Send an invoice.