I need to give access to the IIS user to a pfx certificate. The website is running under the App Pool under some user AppPoolUser. IIS automatically has the user name \"IIS
If you still want to continue using wihHttpCertCfg, then use the below command:
winhttpcertcfg.exe -i <filename> -c <certificateLocation> -a "ComputerName\IIS APPPOOL\AppPoolName"
I had same issue and I resolved it by above command today.