It sounds like encounter is trying to build a clock tree but lacks footprint info for one or more of the types of clock buffers. I think you have to provide a list of clock buffers for, and you should have LEF files for them so it can figure out the size.
footprints are defined by default in later versions...In older versions we need to explicitly define a footprint...You can define it by using "setBufFootprint" command...hope this will help you...