Ranks 16-18 of all the Mercenary AAs that were added in a recent patch aren't flagged to unlock in any expansion. Phinigel recently unlocked CoV, the current live expansion and these AAs are not buyable.
Haven't had any issues purchasing the newest merc aas on accounts with either ToV or CoV as the highest expansion. This is on a regular Live server (Xegony). You have bought ranks 21-22 of Improved Health, Melee Defense or Spell Resistance before trying to purchase the 16-18 ranks? ... because those are prereqs.
Like Cragzop said, check the descriptions - yes that does unfortunately require some reading but it won't hurt you I promise. Most of the useful Merc aa require that you have a certain rank in the more basic lines.
Pretty sure, if they were not flagged to any expansion, then they would have been reported long time ago. bought the aa long time ago. You dont look at current rank, but at next rank, what the requirements are. Example healing acumentt, 16 = level 115 + improved resistance 22(next rank). if look at current, it say lvl 110, isr level 20.
All the merc aa are tied to specific expansions but those expansion requirements are not shown on the merc AA screen, they perhaps cannot be or they would cause a conflict with the filtering system used for your character AA.