The full message is: "No post numbers found in that picture. Use a screenshot of the post or its insights, or type the numbers in." The request was not counted.
You may also see:
"That screenshot could not be read. Try again, or type the numbers in."
"This screenshot is too large."
"Choose a screenshot (an image file)."
"This picture could not be read."
What to do
Take a screenshot of the post's insights or statistics screen, with views, likes, comments and shares visible.
Crop out anything else and try Read from screenshot again.
Or type the numbers. 1200, 1.2k and 2m all work.