Hi @Emma Johnson
If you want to change the user's display name, it is divided into different account types.
Organization Account: you need to contact the administrator to change the display name in the M365 admin center.
Personal account: you can click the avatar in the upper right corner of Teams, then click the profile card to the right of the avatar, and finally click Edit name.
For the error you need to log in with a school or work email address, it may be that the client version you downloaded is wrong and you cannot log in.
If you downloaded the version of Microsoft Teams (work or school), you will need to use an organizational account instead of a personal account.
For more details you can refer to this document
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