Getting unauthorized error even with correct username

Getting unauthorized error even with correct username
I am trying to run Dexter utility on Linux server, am using correct username i.e., "username@customer1" but still it is throwing error.
Please help.

No source file, test, or entry point is named. Start by reproducing the Dexter utility on a Linux server with the username format shown and capture the complete unauthorized error and invocation context. Done would require identifying whether the credential format is accepted and documenting or fixing the authentication failure.

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