Here’s the link to the InvoicePlan FAQ:
https://wiki.invoiceplane.com/en/1.0/general/faq#error01
And here are some threads that are already discussed this topic. Please try the solutions posted there first:
https://community2.invoiceplane.com/t/solved-installation-error/175
https://community2.invoiceplane.com/t/solved-new-install-upgrade-to-v-1-1-not-working/149
https://community2.invoiceplane.com/t/solved-invoiceplane-setup-404error-nginx-ubuntu/1050
https://community2.invoiceplane.com/t/solved-setup-not-working-404-error/205
https://community2.invoiceplane.com/t/solved-404-while-trying-to-run-setup-install/733