Based on the error message that you mentioned, it seems to tell that there is something issue with the Office365.FindMeetingTimes() function in your app.
I have made a test on my side, and don't have the issue that you mentioned. The screenshot as below:
Please check if the Office 365 Outlook connection you created within your app is valid. Please consider take a try to remove the Office 365 Outlook connection from your app, then re-add a new connection to your Office 365 Outlook with a proper Office 365 account (work or school account, rather than your personal account), try your app again, check if the issue is solved.
If the issue still exists, please consider take a try to re-custom a new app based on the Book a Room template app, check if the issue still exists.
Community Support Team _ Kris Dai If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.