why when I want to download viber setup, it says: "This type of file can harm your computer?"?

the above message appears when the application has no permissons
when we download to the mobile, it is downloaded from playstore(genuine website) so above message doesn't appear.
to prove this try to download a viber ".apk" from laptop and send to mobile and try to install from there (provided in the setting should uncheck that app permissions

if you are on the true viber site, should be fine

Apparently the download comes as an "exe" file. That is a file that you can run as a program by itself. Since those kinds of files can also be used to download viruses and other harmful software it warns you that you are about to download a file that could contains something harmful. It does not mean that it DOES contain something harmful. It just COULD.

Make sure that you are downloading the file from the actual website of the maker of the program.