Members are unable to log in to Teams due to pop up window

That's the kind of message you get when you're not having a Teams license assigned for a particular user account trying to connect to a particular organization. I got a feeling these member perhaps are using Teams with other accounts as well? It can be a mess with multiple accounts on a Teams desktop app sometimes as the credentials and other stuff are cached.

Try signing out manually from the top right corner and quit Teams from the bottom right taskbar icon (right-click and quit). Then clear the Teams cache https://docs.microsoft.com/en-us/microsoftteams/troubleshoot/teams-administration/clear-teams-cache

If no luck with that try this script too https://docs.microsoft.com/en-us/microsoftteams/scripts/powershell-script-teams-reset-autostart