Sorry you can not merge accounts. You have to pay for 2018 in that account to unlock it. You don't have to file a 2018 from it.
Just Enter your personal information in the 2018 Free edition. The program will ask if you want the Plus Upgrade. Select the Plus Upgrade.
Did you pay for a 2018 return in the other account? Ask support to unlock it. Contact Customer Service 5am-5pm Pacific M-F
https://support.turbotax.intuit.com/contact/
