I am not able to activate a terminated employee after a rehire

Just add as a new employee.  You will have to change the name, maybe a space before the name.

Let’s reactivate your employee, faogsec.

You'll get an error that an employee name already exists if you create a different profile.

Did you just move your files to QuickBooks Online? Usually, you won’t have an option to reactivate if you moved from a different platform, and you haven’t run payroll in QBO yet. In this case, you can run a dummy payroll for an active employee and reactivate the terminated one. Then, you can delete the dummy payroll after.

Please let me know what happens after you do the steps I mentioned. We’re just right here to help you further.