Here's a list of IRS Efile Error Codes, but it's 36 pages you probably don't want to review.
As @SusanY1 indicates, the IRS computer most likely is programmed to compare Wages to Withholding, and if a certain % is exceeded, automatically rejects the Efile.
They want a person to look at the actual document, hence it must be mailed.
@mja400
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