Font.PLAIN doesn't exist in Greenfoot 3.1.0's Font class (greenfoot.Font).
You need to explain why you think you need it, and/or show how you are using it in your code, before anyone will be able to help you.
If you are referring to the font style modifier, you are correct. The new 3.0 version has changes that affect Fonts.
Make sure you check the method signatures, you may also be using the parameter in the wrong location with the new changes. Always helpful to post a bit of code as well.