This appears to be a conflict between your employer and the IRS - the employer claims the W-2 was sent to the IRS and the IRS says it was not. Either the employer did not properly send it or the IRS is wrong and you are caught in the middle.
Call the tax advocate again and tell them the employer did send the W-2, but the IRS did not list it. and ask for advice.
If you received a letter from the IRS it should have a number to call to dispute it. Or a mail address - send a copy of the W-2.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**