When your visualization has a SQL query applied, which seems to be the case in this scenario, the table is set as "view mode", which means that you cannot edit the contents from that view.
Editing contents stands for editing the values in the cells of your table, but also for adding new points as it's the case.
In order to sort this out, you need to go to the original table (if you're working in a visualization) and add the points there. The changes in the table will be reflected in your visualization if your SQL query matches with the new data added.
Of course, clearing temporarily the SQL query is also another option. :-)
This behaviour and the suggestion to avoid this "blocking" mode is reported in this GitHub issue.