Merge boxes and getting rid of a "Warning" message associated with it.

I have a “Warning” message and the word ‘warning’ pasted on a merge box in the program.
What does that mean? How do I get rid of it? Do I need to?
[Merge 1: rendered rectangle (0,0) , (84-113) contains NaN values. They have been converted
to one] This is the green error message info I am getting. What does it mean if anything?
*How to get rid or this ‘error’ message or issue is what I want to do.
Thanks for any and all responses