Hi,
You can find the explanations about the workshop in this page.
About the long lines, it says: "Length of Clue Lines - The maximum length of a clue line is 20 clues. If a clue line has more than the permitted length the workshop alerts: "Long row - FAILED!" or "Long column - FAILED!" for more than 25 clues and "Long row" or "Long column" for lines with 21-25 clues. The workshop also places circles on the puzzle in the long column/rows."

So it means you've got too many clues in the marked lines, which would make the solving tedious and the puzzle hard to fit to your screen.

I hope this helps!

Jelga