You could create a dynamic text styled as a warning, and hide it (property "visibility") with an expression such
params["nummonths"].value <= 12
There is an example of a such approach here, if we select more than 10 countries or more than 10 indicators, a warning label is displayed at the top of the report.
An interesting point is, although a warning is displayed we can additionally create a rule to replace the wrong parameter value in a script such onCreate. This way the report can correctly run. For example in your case, we could do in a script:
if (params["nummonths"].value > 12){
params["nummonths"].value=12;
}
Alternatively, you could also drop some report elements in "beforeFactory" when the parameter is wrong.