There is no such IRS Form 511 so it must be a state form. Since no one can see your tax return which state program are you using?
Is it maybe IRS form 15111?
IRS Letter 15111
https://www.irs.gov/pub/irs-pdf/f15111.pdf
CP 09 is the IRS Notice that issues the form 15111. Use the link below to the CP 09
https://www.irs.gov/individuals/understanding-your-cp09-notice
And see this post for more info especially page 4
https://ttlc.intuit.com/community/after-you-file/discussion/anyone-else-need-help-with-finding-the-form-15111-got-a-notice-to-fill-out-and-return-the-form-to/01/2061646#M524794
@ryan-miller11 wrote:
Oklahoma
The Form 511 is the Oklahoma Resident Income Tax Return. If you are a resident of Oklahoma and have taxable income you are required to file the state income tax return.