If you are unable to select the State and Country fields under eInvoice, Configuration, Customer, it is likely that the State and Country have not been mapped for eInvoicing. To resolve this, please ensure the State and Country are properly configured and mapped in the system.
Go to Miscellaneous.
Select State and Country.
Locate the relevant State and Country.
Save the changes.
Return to eInvoice, Configuration, Customer and verify that the State and Country.