Simply pasting what you posted into my copy of Matlab gives a hint about the cause:
I get a warning from m-lint (the orange squiggle) that states that "The format might not agree with the argument count".
The reason is that you made the entire input a string, instead of only the format specifier. Do not mix single and double quotes. Either of the versions below will work.
fprintf("afunc*bfunc gives us %d therefore is a root between 0 and 5 \n", rootproof)
fprintf('afunc*bfunc gives us %d therefore is a root between 0 and 5 \n', rootproof)