I've a simple model to iterate through a feature dataset and rename all the feature classes, adding "geo." before the original filenames. The output of Rename tool looks like "geo.%Name%", but running the model i get Error 999999 message, i think it's because of the point after "geo", so i'd like to ask if there is a way to keep the point in the filename, but for the tool to ignore it and run the command?
The error is caused by the point, that is an invalid character to use in a feature class name. More information about Defining feature class properties including naming conventions is discussed on this page.
If you want to separate geo from name then use an underscore.