Hi Nader,
in your picture you can see 2 different types of NaNs (not-a-number):
a. some spots here and there (usually in areas of steep slopes): they correspond to SRTM missing data. Sarproz interpolates those values when the DEM in SAR coordinates is generated. You can check the result after your process it, by plotting it with the “view parameters” module. This is not a problem.
b. half of your picture which is totally blue. If you scroll the log file, I’m sure you will see a message in which Sarproz told you that some problems occurred while downloading an SRTM file. The sw also tells you that, since that files was not available, it was filled with zeros. To force the sw to download it again, delete the corresponding file in the SRTM folder. If you are not sure about which is the problematic file, just try to move one or some of them in an other folder and read the messages given by the software/plot the DEM again.
good work
d