Apple released macOS 10.15 Catalina on October 7, 2019. This new release has stricter security requirements that require explicit permission to allow Splashtop applications to have control and work properly. Same thing for Apple's operating system up to Ventura macOS 13.
Video of setup
See step by step below
1. After you download and then the Splashtop_Streamer_INSTALLER.dmg, you will need to add each permission below under the Security & Privacy in System Preferences.
Our guided window helps open each section. Click OK and then Allow and follow the guide.
2. Then under Accessibility, add the Streamer app by clicking the + sign or toggle Streamer if already there. You’ll be asked to enter your system password and click Modify settings
3. You will process the same for the other two permissions: Screen Recording and Full Disk Access.
4. After that, you will get the success window with 3 green check marks. You should be able to have full control of your Mac.