I can see how this would be confusing.
If the K-1 has 2017 on the top of the form you will need to file an amended 2017 return to report the K-1.
If the K-1 has 2018 on the top, then you would file the K-1 in 2018.
The final return for the business should be filed in 2018 since there was cash that came over in 2018. It can be a zero return to close out the business, but that way the IRS does not get confused.
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