Cannot connect to the VPN even though the credentials are correct

[anuranj]

Every single time, it shows the error "Unable to get VPN permissions, you may need to reboot device and try again". Also in OpenVPN open source android app, the log file shows this error

Your image does not support the VPNService API, sorry
If you did not get a VPN confirmation dialog, you have "Always on VPN" enabled for another app. In that case only that app is allowed to connect to a VPN. Check under Settings-> Networks more .. -> VPNS

It is working fine in my Linux and Windows. What should I do to fix this?