Did you try to use the same tape as the test job?
If so, BackupExec was probably observing the OPP (Overwrite ProtectionPeriod specified for the media set specified.
If you inventory the tape, does the info show up in bkue or black. Blue would indicate an overwritable tape, black a protected tape.
If it shows in black, drag it to the scratch media set.
If the job specified Append only, BackupExec will not accept a tape with no active backup sets, or that does not belong to the media set used by the current job.