So, there is this Outpost Rush bug that’s been out for ever where when the Command Center building has been destroyed by the enemy team and is on a state of ‘‘Fire Burning’’. If it’s on Tier 1 or 2, After repairing it, The option to Upgrade to the next Tier won’t work anymore.
PS: This also sometimes occur on Doors that aren’t fully upgraded. (Upgrade Tier Option stops working).