we are now trying to complete my father-in-law's tax return for 2018.
You have to create a second, completely separate account for your father-in-law. Then, you can begin his return.
Are you using the Online version? The Online version is only good for 1 return. But you can use the same email address for 5 accounts. To do another return you have to set up a new account and new user name and pay any fees again.
https://ttlc.intuit.com/replies/3288574
