The in-game message just says "Install more upgrades".
The only information I can find online is this less than useful note on wikia (bold mine):
NOTE: The last two upgrade 'trees' for both weapons and Fletcher's combat gear are 'locked out' until a number of earlier upgrades are purchased first.
So, specifically how many upgrades do I need to buy before they are available? Also is the threshold on total upgrades, or are there different counters for each of the three categories (CLN, N.R.G, Combat Gear)?