GC-MS 5977 autotune stopped showing “The valid range for EM Volts is 0.0 to 3000.0, Step 0.18”

This error means that the system was trying to turn the EM up higher than the maximum to tune. 

How long has it been since the last full preventive maintenance was performed? Is the vacuum reading normal? Clean the ion source. Make sure that the filaments are both good. Make sure that the column is installed properly into the mass spectrometer as if it is extended too far out of the transferline it can block the electron beam.

(see:(+) Column Installation - Split/Splitless and Multimode inlets =and= SQ and TQ MS transferlines - Files - GC/MS - Agilent Community)   

Verify that there is PFTBA in the calibration vial.  Make sure that PFTBA tune ion peaks can be seen at the beginning of the tune.

Thank you so much! The PFTBA is in the calibration vial; every signal looked perfect at the first several minutes, until the error message jumped out.

It's an old GC-MS we got from somewhere else so we have no idea when the preventive maintenance was performed. the vacuum reading is normal.

Other than cleaning the source and verifying that the column is installed properly, try a different tune file to make sure that the tune file you are using is not corrupted.  If your system has the typical orthogonal ion source, try STUNE, for example.   If your system has an HES, try an older version or reload the default and start from there.