Did you searched the forum for related posts? There are several about the same problem:
https://community2.invoiceplane.com/t/can-not-create-quote-invoice-or-quote-to-invoice/1003
https://community2.invoiceplane.com/t/submit-invoice-hangs-in-browser/1066
https://community2.invoiceplane.com/t/cant-save-invoice-quote-user/1220
https://community2.invoiceplane.com/t/invoices-cant-be-saved/458
https://community2.invoiceplane.com/t/invoices-not-saving/176
Please check them first. If you couldn’t find a solution post the console output as described in several of these posts.