You may see this error in your unipost_d01.00.out files. It is not actually an error and does not mean that your run has necessarily failed. It is more of a warning telling you that particular variables, e.g. HTOP, was not found in your wrfout file. UPP reads through your wrfout file and tries to find all of the possible fields for that model, even if they are not needed in calculation for the fields you request, and displays this warning message to inform the user. As long as that particular variable is not needed in the calculation of your requested output fields in wrf_cntrl.parm, then this error is just a warning.
If you are running UPPV4.0 or later, you will also see additional lines related to that variable not being found.
NetCDF error in wrf_io.F90, line 3345
NetCDF error: NetCDF: Variable not found
