The Text Overlay Extension uses a technology called WebGL, which requires you to be using Chrome browser and have hardware acceleration enabled.
If you are using Firefox, first try switching to Chrome.
If you are using Chrome, ensure that you have hardware acceleration enabled. To do so, follow the instructions below.
1. Open Chrome Settings by clicking the menu icon and then clicking on 'Settings'.
2. In the Settings tab, scroll to the bottom and click 'Advanced'.
3. Scroll down to the System section and find the 'Use hardware acceleration when available' setting. Toggle the switch to the 'On' position and then click 'Relaunch'.
Warning: Make sure to save any work in progress. Chrome will reopen the tabs from before the relaunch, but won't save any of the data contained in those tabs.