Am I allowed to wire a 60a subpanel as 120v only? Two circuits, 15a and 20a will never need 240v. 40 feet from main panel use #6/2 AL? Can I add a ground bar to a Siemens EO2040ML1060 panel?

A 120/240 volt panelboard cannot be used for 120 volt only.

A 120/240 volt systems must be at all panels, so #6-3 with ground cable will be needed.

Yes, a ground bar can be added to any panel.

Look at the data plate on the box. It my say 120V 2 wire OR 240 Volt 3 wire. Just remember to identify the Neutral since a 6/2 will only have black feed lines. I'd also mark the box 120Volt only.

Just remember to identify the Neutral since a 6/2 will only have black feed lines. ??

What kind of 6/2 are you buying?

It says 120/240v- 1 phase 3 wire and also 208/120v- 1 phase 3 wire. It has 2 connectors for hot independent of each other; does that mean I could use a 10/3 giving 30a to each terminal with a 15a off one and 20a off the other? What size ground?

My 70 year old brain was in a resting phase. I meant that in a 240 there is no neutral and SER has 2 black not 10/2

See TK's answer.

I already had the 6/2 NM left over and wanted to use it if I did not have to wire it 240. I know I would have had to label one wire as the neutral.
Thanks all, I'll buy 6/3 and go 240.