Right click the toolbar; select Customize (at the bottom of popup menu)
Open the Commands Tab; in The Categories box (to the left) select Edit
In the right had window locate the Underline icon; carefully drag this
(click on it, hold down left mouse button and move the mouse) to the
Standard toolbar - drop the icon after the I (italic)

OR learn to not use this archaic format which was invented for the
typewriter (now a thing of the past); and use a border to highlight a cell
best wishes