Unable to add columns in Numbers after update

> I read the user guide's section about adding or removing rows and columns,

Not sure what user guide you're referring to

> but the icons I'm told to click on are nowhere to be found on my end

What icons are you looking for?

Off-hand, I can't think of any icon for inserting or deleting a column.

The two classic ways are via the Table menu:

or by clicking the little ⌄ icon in the column header to show the column contextual menu, and choosing to add (or delete) a column from there:

You can also add columns to the end of your table by clicking the || icon at the end of the column headers:

You can delete empty columns on the right by dragging the resize handle at the bottom of the table towards the left (or up to delete rows):

(note: the latter two options only appear when the table is selected, so may not be visible until you select something in the table.

Arrange->Unlock

Your table is locked

Do you have a chart or a table selected? Charts can't have columns added to them, they rely on the data reference.

Yeah - in the context of 'Add Column', that relates to the table.

I see where the confusion comes from - conflating columns in a bar chart with columns in a table, but they're different. Consider that the 'chart' may not be columns at all.. could be a line chart or a scatter chart, or a pie chart, where the term 'column' doesn't really apply.

Instead, as you've found, charts use the term series to denote data to be displayed.

Adding a column to a table does not automatically add that data to the chart. Either collate all your data first, or Edit Data References to add more data to the chart after it's created.