Sounds like you may need to amend your 2018 tax return. A single member LLC is reported on SCH C as a physical part of your personal 1040 tax return. But first:
Back in 2018 or before did you file IRS Form 2553 to treat your single member LLC "like an S-Corp"? Or did you file IRS Form 8832 to treat your single member LLC "like a C-Corp"? If you did either, then you flat out can not report your business income/expenses on SCH C. You are required to report all business income/expenses on 1120 for a C-Corp or 1120-S for an S-Corp. Then (and only then) would the C-Corp or S-Corp issue you any kind of tax reporting document.
