The reason for the error is because
math_counter outputs a "float" (floating-point number, which is fancy computer lingo for a single number with a decimal), while
prop_dynamic's SetModelScale input accepts a "vector", which is a series of multiple values. In this case, SetModelScale accepts two values: one for the new scale of the model, and the second for the number of seconds the model should take to resize itself.
As per how this difference can be rectified? Beats me. No entity that I know of can convert numbers to vectors. A few entities, like logic_lineto, are capable of outputting vectors, but those aren't meant to be general purpose and could be tricky to use, as well as possibly outputting vectors with too many values (which may or may not be an issue).