Hey Jessica,
Thanks for reaching Dynamics community. Is the user a guest Azure AD user on Dynamics 365 tenant? Or this may happen due to the logged in user account within IE uses machine account which could be a part of an O365 tenant however the CRM user account is in another tenant.
Please try to clear temporary internet files or use an incognito window to see if this allows the affected user to log on.
For IE :: Hit F12 to open the Developer Tools, click Cache and Delete session cookie, do the same for cookies.
Please mark this comment as answered if this helps.
Thanks,
Saurabh
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