Just received a letter from Detroit stating that I need to submit a Form 5120 for 2018 and I can't figure out how to get the file. How do I generate this document?

Detroit, Michigan city tax returns are included in the TurboTax state program for the state of Michigan.

Were you a part time resident of Detroit in 2018?  If so, you should have completed the city return when you were using the TurboTax 2018 state program for Michigan.

Form 5120 is the City of Detroit Part-Year Resident Income Tax Return (2018) - https://www.michigan.gov/documents/taxes/5120_642095_7.pdf

Form 5120 Instructions (2018) - https://www.michigan.gov/documents/taxes/5120_instructions_642111_7.pdf

When I download the full 2018 tax return file in my documents, I don't see a 5120 in there at all. If I failed to somehow do it when I originally filed, do I have to go back to 2018 and use the amend return feature to get one?

Download the amendment program then revisit the state interview ... you must have missed something that generates the form you need.  

Or you can simply fill in the form ... the link is in the other answer. 

Better check your 2019 and 2020 returns.   If you made that same error on those returns, the city of Detroit might not have caught it yet.