Introduction:

This issue occurs when the sales order is integrated with the third-party application.
when the Back office processing integration is enabled then create invoice and fulfill order button will not be visible and the new submit button will be visible

Resolution:

Go to App settings -> Overview -> Back office order processing integration -> Manage.

Clicking on Manage gives you the option to disable sales order processing that is integrated with another application such as Business Central. Back Office Order Processing integration is disabled. Then you can see that Create Invoice and Fulfill Order button and Submit Order button will be disabled.

Note:

The Back Office Order Processing Integration setting appears only if Sales Order Processing is already enabled. After the setting is disabled, users will no longer be able to see this setting. This setting can only be used to disable sales order processing if it has been inadvertently turned on through a solution installation.

submit order button:
When a user clicks on submit order the order status is set to submit. It is then integrated with an external order processing application where the lifecycle of the order continues.

Can we create an invoice if integration is enabled?
Ans:  You can’t create the invoice if integration is enable.  But you can create the invoice using the custom button and action.