Internal check system error usually appears when you have an endless loop in your code and our checker gets "stuck" in it.
If you have checked your code and everything is correct, please archive the affected Hyperskill project folder, upload it to the file-sharing service and attach the link to this support form. Don't forget to include a link to your profile and a problem or project stage you're trying to solve. It will help us to understand the issue and process your request much faster.