After installing Picsee browser extension, if you use it immediately, a dialog "Picsee is not open" will pop up. This is because Picsee browser extension must be using with Picsee APP, which must be installed and opened firstly...
If you installed Picsee on macOS, you have installed the extension. The only thing you have to do is to make sure you have enable Picsee extension in Safari Preferences and confige right. ......