I can see the impact of this login issue on your business, @Khurram1010. Let's ensure you're able to sign in with ease as soon as possible.
Make sure you're on the right login page using the correct credentials for your QuickBooks Online company.
If you're still getting the same error message, I suggest performing some basic troubleshooting procedures to check if you're having a browser-related issue. Sometimes, historical data can cause this kind of problem.
Let's start by accessing your account using an incognito window. Refer to these shortcut keys to open it: