Trouble with Static Mode

[QBeamus]

I've got the 16th Army screening Lennigrad's isthmus, so I want to put them in static mode. I've been building fortified units and assigning construction detachments to help build up the fortifications, despite the swampy terrain. Now that some of the hexes are reaching fortification level 2 I'm clicking on the units, but the option to assign them to static mode is not showing up the way it shows it should in the manual. Any ideas what the problem could be?

A related thought: Having the UI contextually appear and disappear does not seem like a great idea to me, because it makes learning to navigate the system harder. It seems to me it would be better if the "static mode" button was always present, but was greyed out, or otherwise visually indicated that the option was presently unavailable when it's not. Ideally, it would even have a pop-up that explained why it was unavailable, though I'd understand if it worked like the "motorize" button, where it only devotes the processing power to calculating whether you can actually do it when you actually try to push it.

Thanks in advance.

[Sarge11]

I could be wrong so please correct but your units have to be in place for a certain amount of turns 3? before staic mode is available

[homer82]

Units can't be set to Static Mode in 1941. Perhaps that's it?

[QBeamus]

Units can't be set to Static Mode in 1941. Perhaps that's it?

That's it. Thanks!