While attempting “Land Development” I get an error message “Cannot March, insufficient Combat..

Combat command is a thing that works like APC durability, that error message means your combat command is less than 200 (the requirement for land development)

It says that the combat command resets daily, but I'm not sure how much as I don't keep track of it

I just had the same issue except I was able to update 14 hours ago, now I can't. Crazy.

Meaning you aren't strong enough to take the tile.

If you're in the DoomsDay event then you do need to be strong enough as well as be immune and or with enough troops to do so. You can get stronger by upgrading your Honor Structures - Assault Fortress, Guardian Fortress, Virus Research Facility.