Use "wall" there.
Student or Learner
“The writing is on the walls” or “The writing is on the wall”.
I don’t know if I split hairs but I know in some idioms putting “s” at the end of a sentence would be an error, not sure if this is the case here. I have never used this idiom, nor have I seen it, until today (with “s”). I could not find anywhere the "s" ending.
He said that the most severe problems were found in isolated, single doctor practices. "The writing is on the walls" for these type of practices.
It's an allusion to the Biblical story in the book of Daniel.
The whole story appeals to my imagination, thanks, and this in turn helps me a great deal in memorizing new words and phrases. Excellent way of learning idioms; I must say, no matter how difficult they may seem at first.