Let me sure some information so you can successfully import your QuickBooks Mac company file to QuickBooks Online.
Just to clarify, are you getting the error prompt while trying to change your QuickBooks Online account while you're in the middle of the exporting process? If so, let's first make sure to update your password using the regular browser outside QuickBooks Mac. Also, you'd want to ensure that you logged as the Administrator in both your QuickBooks Desktop for Mac company file and your QuickBooks Online account. This way, you'd be able to successfully start the importing process.
Once you're able to successfully sign to your account, you can now start the importing process.
From your QBDT Mac, go to the File menu.
Go to Export to QuickBooks Online.
In the pop-up window, sign in to your existing QBO account credentials.
Once done, hit OK, got it to complete the process.
You'd receive an email confirmation once the importing process is complete. This may take up to 24 hours. For more information about the process, consider checking out this article: Converting a Desktop for Mac file to Online.
If the error persists, I'd recommend Technical Support Team. A specialist will securely look up your account and further assist you with the importing process.