Problem
The problem is that an inactive account is already using the email address so when adding this email to an active account, the user gets an error.
Solution
To solve the issue, edit the inactive account and change the email. Use the Change the email in the inactive account workflow to change the email.
Note: Only admin users with the View Or Update User Employee Permissions permission will be able to remove the email address from the inactive account.
Change the email in the inactive account
In ServiceTitan, go to the navigation bar and click Settings.
From the side menu, select Technicians if the email address is in an inactive technician account or Employees if the email address is in an inactive employee account.

Click the Active column and select Inactive.
Click Filter.

Select the inactive technician or employee and click Edit.
The Edit Technician/Edit Employee page opens.From the Email field, delete the current email and put another unique email address which is not connected to any account in ServiceTitan.
