If the user is already there in UAT Sandbox then ask the user to use UAT sandbox username and production Password and if they forgot the production password then you can reset the password. If the user is not there then create a new user and give access to them.
If the user is already there in UAT Sandbox then ask the user to use UAT sandbox username and production Password and if they forgot the production password then you can reset the password. If the user is not there then create a new user and give access to them.
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