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