Hi Nev,
Thanks for the reply and the screenshot.
It has to do with the type of table you are inserting, I can see that you are inserting a To Do task list and not a simple table, which is why such tables do not support custom columns.
The inserted To Do task list form does contain only four fields by default: task name, assigned to, due date and bucket. this is designed to simplify task management and ensure that users can quickly create and assign tasks. The option to customize adding more columns is indeed not supported at this time.
If you press the / key and click on Table to create a simple Table, the columns can be added using the method mentioned in the image I have given.
If you think this provides useful information, click "Yes" or "No" to tell us how you feel and it may help other users with similar issues.
Please feel free to ask any questions!
Best regards,
Ian - MSFT | Microsoft Community Support Specialist