Too short
A check needs a few sentences to say anything useful; the message names the minimum. Add the rest of the passage, or paste the whole section rather than a single line. The notice clears on its own as you type once the minimum is reached.
Too long
One check covers up to 25,000 words. Free scans without an account cover 250 words. For a longer document, split it by chapter or section and check each part; the reference list can go with the last part so its citations are checked once.
The count looks wrong
Word counts follow the text as extracted: a PDF with a table of contents, running headers or footnotes on every page counts all of them. Removing that scaffolding before the check makes the count, and the credit cost, match the writing itself.
Empty paste
A paste that arrives empty usually came from an app that copies formatting without text (some PDF viewers, some note apps). Paste into a plain text editor first, then copy from there.