With MacOS Mojave (10.14), Apple requires that you grant explicit permission to remotely control the computer.
- Open System Preferences -> Security & Privacy -> Privacy
- Select Accessiblity
- Click the lock to make changes
- Check Splashtop Streamer
- Click the lock to prevent further changes
This must be done locally at the computer or from another tool that already has the accessibility permission.
The video below will walk through how to fix this.