You need to follow the instructions from the IRS letter.  It should explain what you have to do.  Whether that means send in the missing form, or it may say amend your return to include the 8995 Form.

For more information on the 8995 form, see the following:

About Form 8995, Qualified Business Income Deduction Simplified Computation

https://www.irs.gov/forms-pubs/about-form-8995

Also see, 

What is Form 8995-A?

https://turbotax.intuit.com/tax-tips/irs-tax-forms/what-is-form-8995-a/L0N1SP8vU

Save a PDF of the return and look for the 8995 ... if you have one print it off and send it in as the notice directs you to do.  

what year? one op posted that the IRS wanted it for 2018 - the op filed late. the problem was the IRS did not have the form until 2019.