I can’t turn off vpn on ipad

Are you using a VPN App - or have you simply manually configured a VPN connection in Settings?

If you are using a third-party VPN App, there is strong possibility that you have configured Connect On Demand somewhere in the individual App settings. This configuration setting may be worded as something else by the App (e.g., Kill Switch, Persistent Connection etc.) but it will have the same effect. 

When so configured, the VPN App will try to ensure that your network is always protected by the VPN - as such, it will restart the VPN if has disconnected for any reason. When this occurs, you’ll see the VPN icon appear - and possibly the VPN App.

For VPN connections configured directly from iPad settings:

Settings > VPN > [Selected VPN Profile] - tap the blue "i" icon - and ensure that Connect On Demand is disabled

Be aware that if you have a centrally managed device that belongs to an Enterprise/Business, or some Educational establishments, the VPN configuration might be required by the applied device management policy. In such situations you may need to seek assistance from the IT Department or Administrator that manages your device.

I remember NordVPN doing this on my iPad. I believe rather than using the VPN button on the iPad I had to turn off the VPN with the app.

Contact your VPN for assistance.