If the 1099-K is for self-employment income, and you can't get it to link to your Schedule C, you should delete the entry and enter it directly in the self-employment income section. You may have it entered more than once also. Use the instructions below to delete the 1099-K(s) and then use these directions to re-enter the 1099-K directly on Schedule C. If you use the self-employment entry, you get the screen below, and you won't need to link it.
How to delete forms in TurboTax Online
How to delete forms in TurboTax Desktop