This refers to Moodle 3.10 from August 2021 onwards
Creating a table in the Atto text editor
Inserting a table in a Label or Page is simple and includes options to add a border and grid lines.
- To create a table click the 'Table' icon in the text editor toolbar

Give the table a caption or title, say whether you want to put headers on columns, rows or both, and choose number of rows and columns.
You can choose how you wish to format the table borders and background colour.

It is not possible to format the table with merged cells or in any other way in the ATTO editor.