BoardPro Notes is only available with the Premium or Ultimate Plans. If your board subscribes to the Essentials Plan, BoardPro Notes is not available.
It is the stage of a Meeting which determines when BoardPro Notes is available.

Available stages:
Published Agenda
Draft Minutes
Minutes in Review
When in the full version of BoardPro, you can see the Meeting stage on the Meeting card:

and if you are in the Meeting detail page, it is here:

Related Articles:
Meeting Stages and Flow
BoardPro Notes Demo