Received a Notice from IRS

2021 return. Filed electronically and accepted on April17.

Married, filing separately, we both file with standard deduction $12,550. 

Taxable income $34,000. Receive SSI (I'm 64) and have a pension plus, work a part-time job. 

Notice said that my standard deduction was reduced because my income did not exceed the standard deduction or I was claimed as a dependent by someone else. Now own more taxes. 

Not sure why there are 2 possible reasons.

I believe  I was fraudulently  claimed as a dependent (latter part of the reason) as my Taxable income is $34,000.  

Recommendation  on how to best  resolve this issue?