Help binding actions for mobile?

Your general usage of ContextActionService isn't the issue, there are a couple of errors within your function that are causing the issue, I have labelled them in my quick re-write as I debugged it.

local CAS 		=	game:GetService("ContextActionService");
local plr 		=	game:GetService("Players").LocalPlayer;
local char 		=	plr.Character or plr.CharacterAdded:Wait();
local hum 		=	char:WaitForChild("Humanoid");
local skills 	=	plr:WaitForChild("Skills");
local speed 	=	skills.Speed;
local invis 	=	skills.Invisibility;

local cooldown 	=	false;

function useSkillMobile()
	if (not cooldown) then
		if (speed.Value == true) then -- We need to reference .Value when referring to a BoolValue object;
			hum.WalkSpeed 	=	24;
			cooldown 		=	true;
			wait(5);
			cooldown 		=	false;
			hum.WalkSpeed 	=	16;
		end

		if (invis.Value == true) then -- We need to reference .Value when referring to a BoolValue object;
			char["Left Arm"].Transparency 		= 	1;
        	char["Right Arm"].Transparency 		= 	1;
        	char["Left Leg"].Transparency 		= 	1;
        	char["Right Leg"].Transparency 		= 	1;
        	char["Torso"].Transparency 			= 	1;
        	char["Head"].Transparency 			= 	1;
        	cooldown 							= 	true;
        	wait(5);
        	char["Left Arm"].Transparency 		= 	0;
        	char["Right Arm"].Transparency 		= 	0;
        	char["Left Leg"].Transparency 		= 	0;
        	char["Right Leg"].Transparency 		= 	0;
        	char["Torso"].Transparency 			= 	0;
        	char["Head"].Transparency 			= 	0;
        	cooldown 							= 	false;
        end
    end
end

CAS:BindAction("Use Skill", useSkillMobile, true, "x");