Problem checker access depends on your plan.
Writing code can be difficult. The problem checker will analyze your code and highlight bugs or mistakes. Access it via the "Problems" tab in the bottom right corner. Click on any problem to be brought to that section of code.
The Problems tab automatically updates every time you stop typing. You can see where problems exist in your code from the red and yellow squares on the Editor scroll bar. Problems are likely to cause rendering issues while warnings are recommendations to improve code quality.