OPPO and other models will pop up a pop-up window asking whether to allow the creation of a VPN connection when using the accelerator for the first time. If you select "Do not allow" and check the "Do not ask again" option, you will not be able to obtain VPN creation permissions when you enter the accelerator.
Taking OPPO mobile phones as an example, you need to click Security->App permissions->Create VPN connection->Allow in the settings
It may also be that the VPN is set to always on, causing the VPN authorization pop-up window to fail to pop up. How to deal with it: modify the VPN mode in the phone settings and turn off the corresponding VPN always on