MicrosoftKatieZellnerI have seen this a lot when conditional access rules are in place in the O365 tenant as Ilya Bukshteyn indicated. You can check to see if Conditional Access policy "Baseline policy: Block legacy authentication (Preview)" is enabled. If so you will need to remove that as "Legacy Skype for Business" access is used.
If that is not the problem, I have seen where on-prem Skype works just fine, but access to Teams is blocked and it is due to a proxy server that is enabled in the environment and blocking access to the tenant. I wrote an article on how to enable the proxy on MTR's and the link to the official documentation is in the article as well.
These are the most common issues that I have seen with the cannot sign into Teams message:
1. Conditional access
2. Proxy where everything else (exchange, Skype) is on-prem.
3. Licensing
4. DNS
5. Account conig
If you use the account on a browser from the admin mode on the MTR unit, it can expose some of the common issues that I have seen like account config, proxy, DNS and licensing.
