What do I need to do when it says insufficient durability after March failed?

You just need to wait. The APC (formation vehicle has a max of 100 durability and most actions (marches) use up 10 each time. Once you run out you just wait while it regenerates.

It is always regenerating anyway when it is less than 100 but it is slow. So you need to wait a few hours.

If you are in a hurry you can also tap on the Barracks building for the APC and choose the repair option. To repair you will need to buy tools that let you add more durability immediately. You have to pay money for these.