The RINEX parser used by RT Post-Process is not able to handle L5 data, so these must be removed from the RINEX file before it can be used by RT Post-Process. To do this, please download and install RTKLIB, which contains the RTKCONV app that can perform this task.
To reprocess your RINEX file, in RTKCONV, first navigate to your source file by clicking the button ringed in red, then specify where to save the output file by clicking the button ringed in blue.
Now click the options button and set as shown below.
Click OK to return to the main window, then Convert to start the conversion process. The resulting file will be suitable for use in RT Post-Process.