when you click on central buildings of post, you will see two part what you can upgrade.
Upper part are Guild outpost, lower part are for command HQ.
First you must upgrade Guild outpost to +3,+4 .... etc, it depend on how many buildings you have in outpost, and then, only then, you have open option to upgrade command HQ
But we have already upgrade Command HQ to some level (I won't say what so enemies will not know it ) and in this level we have upgrade it is stuck, I mean the button "upgrade" is missing while before it was available
Dark, if it's stuck, you must one more time to upgrade guild outpost, and after that you can upgrade command HQ again.
Edit reason: Double posting, so I merge the replies, nothing changed. Crassus