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 in the full version of BoardPro, you can see the Meeting stage on the Meeting card:
and if you open the Meeting you can find it under Board Pack on the top right and then select BoardPro Notes:
Meeting Stages and Flow
BoardPro Notes Demo