How can I make the email field optional when adding a client?

The email field isn’t inherently required — its requirement depends on the billing method. If it includes sending by email, the system needs an address to send to, which is why the field becomes mandatory. Switching it to print removes the required mark, letting you save the client without entering an email.

For a single client: from the add client screen, in the account information section, change the billing method field to “Print“.

For all clients, or if the billing method field doesn’t appear on the client form: this means the setting is configured at the account level. Change it from Sales → Sales Settings → Invoice Settings, and in the invoice issuance section, select “Print Only” in the billing method field.

Keep in mind that selecting print means the system won’t send invoices to this client by email, and that making the field optional doesn’t stop you from entering an email if you’d like to save one.