How to access VPN which are having PPTP Protocol....on Mac OS Sierra

PPTP is a very old VPN protocol which has been shown to have significant security weaknesses such that it can easily be hacked. As such no sensible organisation would use it any more and in order to help 'encourage' users to use common sense Apple have now completely disabled support for it.

If you know anything about WiFi you could consider PPTP to be the equally bad equivalent to the ancient and also no longer supported WEP encryption standard.

If your company is happy to be hacked they should indeed carry on using PPTP. See https://www.comparitech.com/blog/vpn-privacy/the-pptp-vpn-protocol-is-not-secure -use-these-alternatives-instead/

It might just about be justifiable for a home user to use PPTP still purely for the purposes of remotely accessing their own home systems but with the clear risk of being hacked. It is certainly not justifiable for any business or education or government organisation to use it and arguably doing so could render them liable to legal prosecution for being criminally negligent with regards to computer security under various countries data protection legislation.

Any IT Manager and their CEO, CIO or equivalents should be sacked for incompetence if they still use PPTP.

Ask the owner of the VPN server to accept L2TP connections. You can't setup PPTP on your client macOS Sierra Mac anymore.

The Shared Secret in your Mac's VPN settings needs to be the same Shared Secret that is setup on the VPN server.

The L2TP settings should be:

Server Address: <VPN server>

Account Name: <account you are logging into the server with and that is setup on the server>

Authentication Settings:

User Authentication - Password: <account's password, for the Account Name above>

Machine Authentication - Shared Secret: <shared secret from the VPN server>

PPTP isn't supported any longer in macOS. Use L2TP instead. I assume the VPN server you're connecting to is setup to accept L2TP authentication.