How do I resolve the "This username may be incorrect. Make sure you typed it correctly. Otherwise, contact your admin." error

Dear Azure B

Welcome to Microsoft Community.

Here we see that you clicked on the merchant information by mistake when you started 365 on your new computer, and now your Outlook mailbox can't be used.

Here would like to and first check with you on a couple of issues:

1、Do you tell the following what version of 365 is installed on the new computer? (You can open any word or Excel, click File > Accounts, the upper right message shows the use of the version of 365)

2、Is the account subscribed to 365 products Microsoft account mailbox (Outlook or Hotmail) or other accounts mailbox (Gmail or Yahoo and other mailboxes)?

3、You use the computer is windows 10 or 11, how many OS components? (You can click "Start" > "Settings" > "System" > "System Information" to check.)

Since the information you provided is not complete, we can try the following options based on your question:

1. Confirm the account in your computer:

• Click Start > Settings > Accounts.
• Go to "Access Company or School Resources" and check under "Add a Company or School Account" to see if you have an account. If you do, click on Accounts and then click on Discontinue Account on the right (note that you will need to make it clear that your computer is for personal use before doing this, if your computer is not in use). (Note that you need to make it clear that your computer is for personal use, if your computer including the account needs to be used in the company's domain environment, you cannot interrupt the operation and you need to contact the enterprise administrator to deal with the existing problems.)

-Go back to "Accounts", click "Email Domain Accounts", find the account that started 365 under New Accounts, click and select "Remove".

(Note that if you have added mailboxes in 365, the following process must not be operated, directly skip to step 3)

2、Clean Outlook registry

Disclaimer: Generally, modifying registry subkeys or work group is intended for advanced users, administrators, and IT Professionals. It can help fix some problems, **however, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For further protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.**For more information about how to back up and restore the registry, click How to back up and restore the registry in Windows - Microsoft Support

• Press "win" + "R" and type Regedit, or type Regedit in the search box to open the program.
• Follow the path: (HKEY_CURRENT_USER\Software\Microsoft\Office)

-Mouse right-click the whole "Office" file and export the backup, after the backup is finished, remove the "Office" file from the registry.

3、Remove Reinstall 365 when finished, open any product (Word or Excel) when finished, re-login to your account and reboot, and use Outlook to add a new mailbox when the boot is complete.

Feel free to post back if you need further assistance.  

Best Regards, 

Chris.S-MSFT | Microsoft Community Support Specialist