I gave a user global permissions of View on Invoices. The user is not able to see the Transactions subtab on the Sales tab of Customers. I'm assuming it's permissions related. I don't see any relevant permissions on the Customer form.

Try Find Transaction/Transaction Search permission something like that if that will work for you considering the other implications.

I added that and Financial History and it appears to be working. https://followingnetsuite.com/2018/08/10/netsuite-missing-subtab/