Someone was struggling to log in to Teams via a web browser. After doing a sanity check that they did indeed have a Teams licence in the 365 portal, and they had tried running it in an InPrivate/Incognito window (just in case), further investigation showed that the Web Client was actually set to disabled in the 365 portal (Teams itself was enabled, just the web client wasn’t – go figure!). You can see the current state, and amend as required, via a quick Powershell command (or two):
Launch Powershell and then connect to MsolService
Check current status of Teams Web Client
(Get-MsolServicePrincipal -AppPrincipalId 5e3ce6c0-2b1f-4285-8d4b-75ee78787346).accountenabled
If it returns ‘False’, the web client is disabled so you should enable it via
Get-MsolServicePrincipal -AppPrincipalId 5e3ce6c0-2b1f-4285-8d4b-75ee78787346 | Set-MsolServicePrincipal -AccountEnabled $true
If this doesn’t fix your issue, or you prefer working through the portal GUI, a comprehensive set of suggestions have been provided by Aaron Guilmette of undocumented-features.com.