This can happen for a couple of different reasons. Which access denied screen do you see?
Access to meeting notes is dependent on channel access in Slack. If you are in the channel or group where the meeting was conducted, you should be able to see the notes. If not, the notes will not be visible to you.
When you run a meeting, if you want other users to be able to access the meeting notes, make sure you run the meeting in a channel they have access to.
This is caused by a permission setting within Slack.
Slack allows admins users to restrict who can install and use external applications. In order to allow access to Howdy's note pages, this setting must be changed.
Click here to view the permission settings within Slack. They will look like this:
Either change this so that all users can install and use apps, or add additional people to the list of allowed users. You can read more about that here.