When I try to authenticate the user in the salesforce box setting below the error throw.
" You can not set the Box service account to the same account as an existing user. "
Question
You can not set the Box service account to the same account as an existing user.
https://support.box.com/hc/en-us/articles/360044195713-Installing-and-Configuring-Box-For-Salesforce
Please see the above documentation on setting up the Salesforce integration. It goes over the issue you are having!
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