To move a column, click on the column label to select the column. Insertion points appear wherever the column can be moved. In the example, column B cannot be inserted after column D because column B contains cells with a column span that would require at least one further column to the right.
To move a row, click on the row label to select the row. Insertion points appear wherever the row can be moved. In the example, row 1 cannot be inserted after row 3 because row 1 contains cells with a row span that would require at least one more row below.