Finally I was able to install SSL VPN-Plus Mac Client in macOS 11.2.1 Big Sur!
1) Check SIP is enabled using command: "csrutil status". It needs to be enabled (idk why)
2) Allow Team ID in the recovery mode, using command: "spctl kext-consent add KS8XL6T9FZ"
3) Install VPN using command "sudo installer -pkg SSLVPN-Plus\ Client.pkg -target /"
I finally was able to install the client in my iMac running Big Sur
1) Fresh install macOS Big Sur (11.3.1).
2) Try install VPN Client 6.4.6 (x64) (double click the installer and try to install normally).
3) The "System Extension Updated" window prompts. Click ¨Open Security Preferences¨.
4) The ¨Security & Privacy¨ window opens. Click on ¨Allow¨ to permit the ¨Mattias Nissler¨ extension.
5) ¨A restart is required before new system extensions can be used¨ prompts. Click ¨Not Now¨ and wait 30 secs.
6) VPN Client installer will fail (that ok, keep reading).
7) Restart the Mac directly into recovery mode (press and hold Command + R when restarting).
😎 In the recovery mode, enter into the terminal and type ¨csrutil disable¨ and press enter.
9) Type ¨reboot¨ and press enter. Be fast and go directly into recovery mode again.
10) In the recovery mode, enter into the terminal again and type ¨spctl kext-consent add KS8XL6T9FZ¨ and press enter.
11) Type ¨spctl kext-consent list¨ and press enter. Check that KS8XL6T9FZ is listed.
12) Restart the Mac normally.
13) Copy the VPN installer files into your user home folder.
14) Open terminal and type ¨sudo installer -pkg INSTALLER-LOCATION.pkg -target /¨
15) VPN installer will fail, saying that Kernel extension cannot be loaded (that ok, keep reading).
16) Restart the Mac again directly into recovery mode.
17) In the recovery mode, Enter into the terminal and Type ¨csrutil enable¨ and press enter.
18) Restart the Mac normally.
19) Try again to install VPN Client using terminal. Type ¨sudo installer -pkg INSTALLER-LOCATION.pkg -target /¨
20) You will get some folder permission prompts, press Ok to those prompts.
21) Now VPN Client should install.
Hope it helps!