Hello,
There are a few things that can be checked:
- Confirm the external invite contains a meeting URL - should be obvious (the option "Teams Meeting" should have been activated when creating the meeting from Teams or Outlook)
2.Review Microsoft third party join article, confirm the Exchange calendar and device are configured to allow
https://learn.microsoft.com/en-us/microsoftteams/rooms/third-party-join
3.If the join button is still missing, it could be due to an email security system (e.g. ProofPoint, 365 threat protection). These commonly re-write web url's and can format them in a way that makes the Teams Meeting Room unable to interpret the link and create the JOIN button. If this is the case, you may need to get with your email/security team and inquire about whitelisting the meeting provider (e.g. .webex.com, .zoom.us etc..)
--If the reply is helpful, please Upvote and Accept as answer--