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.
Minutes in Review
When viewing the Meeting list, you can see the Meeting stage on the Meeting card:
If you open the Meeting you can find it under Board Pack on the top right and then select "View & Take Notes":