If only one of the tower buttons is greyed out, then there’s a good chance that it is not set to be compatible with the said tower. Whenever a tower function button is added to viewport, it checks the ‘Activated’ variable (passed in as part of construct event) to determine if the function is available for the selected tower.
In order to ensure that a Tower Function is compatible with a certain type of tower, just open up associated element within the ‘Tower Functions Array’ (in BP_GameInstance blueprint), & add the tower type enum to the ‘CompatibleTowers’ entry.
Let me know if that fixes the issue for you. If not, we can try debugging the system.