The frame does not close with the "Close" button

This might be an issue with Roblox. I would try restarting studio.

yes, to work with all frames at once

From the screenshot it looks like Inventory is your main frame. Is that correct?

I’ll be adding more frames, this is just the beginning

Is the screenshot inside StarterGui?

yes this screenshot inside startergui

Have you tried using print statements to see if the code is being executed?:

local function CloseFrames()
    print("function executing!")
	for _, frame in Frames:GetChildren() do
        print("getting children")
		if frame.Visible then
            print("if statement passed")
			TweenService:Create(Camera, info1, {FieldOfView = 70}):Play()
			TweenService:Create(Blur, info1, {Size = 0}):Play()
			frame.Visible = false
		end
	end
end
for _, frame in Frames:GetChildren() do
	frame.Close.MouseButton1Click:Connect(function()
        print("click detected!")
		CloseFrames()
	end)
end
local function CloseFrames()
	for _, frame in Frames:GetChildren() do
		if frame.Visible then
			TweenService:Create(Camera, info1, {FieldOfView = 70}):Play()
			TweenService:Create(Blur, info1, {Size = 0}):Play()
		end
		frame.Visible = false
	end
end