Hi @Braian Anderson,
Thank you for posting in this community.
Based on the information you provided about the problem, it seems that the problem is on your login credentials, please follow these steps to gradually narrow down the cause of the problem:
1.Please check the account of user is similar to user@yourdomain.onmicrosoft.com. (I know it's simple, but sometimes the wrong suffix can cause this problem.)
2.Make sure that your users have access to the site, you can check the user's permissions according to this article: 2 ways to see what users have access to in SharePoint.
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
3.Multi-Factor Authentication (MFA):
If MFA is enabled on your user, then you can refer to this article to use MFA to log in to your site:
Connect MFA enabled SharePoint site using CSOM
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
Or you can choose to turn off MFA for your user: Turn off per-user MFA.
4.Network Issues: Check if there are any network issues that might be preventing your client from reaching the login server. This could include firewall settings or proxy configurations.
5.Are other users with same permissions facing the same problem? View through another computer or account.
6.Whether all SharePoint sites have the same problem? You could create a new SharePoint site to check.
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