What do I do if a client never sends me a 1099 for work I did?

Report the income anyway. You owe tax on all your self-employment earnings whether or not a client issues a 1099-NEC, and the form is just a copy of what they told the IRS. Clients must send 1099-NEC forms only when they pay you a certain threshold the IRS sets (currently $600) for services, and payments made through cards or platforms are reported on 1099-K instead. Keep your own records of every payment so your reported income is complete and accurate; the IRS matches forms against your return but treats your own books as the source of truth. If a 1099 you do receive is wrong, ask the client to correct it. Never omit income just because no form arrived.