As you can see below, no option SSL Decryption Certificate is avalable.
This is due to the fact that no Content Security Package is uploaded on USG. If you check System > System Upgrade, no package is loaded for Content Security Component Package.
The solution is to go at http://sec.huawei.com/sec/web/freesignature.do. Fill-in the fileds according to your device and download the specific package for your software version, in our case is V500R001C30SPC100.
After we dowload the package, we need to upgrade locally the USG.
After, SSL Decryption Certification option is available.