A compile error is nearly always one of five things, and none of them take long to check. CircuitBlocks runs in your browser at code.circuitmess.com, so there is nothing to download, update or reinstall.
Look for an empty socket. A block with a required input left blank will stop the whole sketch. Follow the error back to the block it names, and check that every block is snapped into the one above it rather than left floating on the canvas.
Reload the editor. Close the tab, open code.circuitmess.com again and reopen your sketch. CircuitBlocks saves your work to our servers as you go, so reloading does not cost you your sketch.
Check the browser. CircuitBlocks needs a desktop Chromium browser: Chrome, Edge or Opera. Safari and Firefox cannot talk to the hardware, and phones and tablets are not supported. There is more in Does CircuitBlocks work offline?.
Check the connection to the device. Use a USB cable that carries data rather than a charge-only one, plug into the computer directly rather than through a hub, and make sure the device is switched on and awake before you run the sketch. My computer doesn't recognise the device when I plug it in. What's wrong? goes through this in detail.
Run a built-in example. If an example runs and your sketch does not, the problem is in the sketch, and rebuilding it a few blocks at a time will show you where it went wrong. If the example fails too, it is the setup rather than your code.
If it is still failing, message us here, or email [email protected], with a screenshot of the whole error message, which device you are using, and your browser and operating system.