Thanks for the post! The maintenance mode message in this instance is a somewhat generic error that typically appears if there is a client-side or browser issue occurring. Generally this means that there is something in the browser (usually an add-on, extension, or script) that is conflicting with the TestRail function.
For now, I would recommend trying the import process again using either a different browser, or in an incognito window to see if that helps move forward with your import.
If you continue to have issues with the import please reach out to the support team directly at email@example.com with any additional details about your installation (Cloud or Server, and if Server what version) as well as screen shots of the error that is appearing and at what point in the import process it shows up and we’ll be happy to assist.
Hope that helps!