Your cursor looks kinda big there. Or is that a heading indicator? (first screenshot) What theme, css snippets, plugins are you using?
Try in the Sandbox then all of the troubleshooting steps in your vault (linked in the template when you created this topic), and report back.
The markdown should be:
- [ ] that doesn't work
with a space between the brackets. There’s a command (Toggle checkbox status) and hotkey set to cmd/ctrl + L by default. You could try that as well.