Ayanna is installed. One last step:

System Settings has opened at Keyboard. There:

1. Under Text Input, click Edit… next to Input Sources.
2. Click +, choose Sinhala, select Ayanna and click Add.
3. In any app, switch to Ayanna with the 🌐 / fn key or the input menu in the menu bar, type mama and press space.

Make your own keys from the input menu › Ayanna Settings…
If Ayanna isn't in the Sinhala list, log out and back in once.