Can someone help me?

Alright, first of all, put a wait in the function... I suggest something like wait(1), just within the function block, then, you may want to convert the message to String.lower. I put some code below to give examples of both. Of course if you chat "Holo Open" exactly with capitalization it you won't need String.lower. Also, connect(open) won't work... you need an event and a colon before putting connect. open() should work fine for calling the function.

--not the full function here
nP.Chatted:connect(function(msg)
lowermsg = String.lower(msg)
if lowermsg == "holo open" then
open()
end

----------
--full function
function open()
Door.CanCollide=false
Door.Transparency=1
wait(1) -- door will stay open for 1 second, use teleportation if you want to safeguard door from those who may get in when the owner goes in in that 1 sec time frame
end
Door.CanCollide = true
Door.Transparency = 0
--reset door if you want

There's no such thing as connect -- ScriptAnalysis should warn you about that.

If you want the door to open, just call open().

You also need appropriate ends to match the if, for, and two function blocks you started (and also two closing parenthesis).

It's good practice to put space around symbols, Transparency = 1 instead of Transparency=1, _, v instead of _,v: it makes it much easier to read.

I would also suggest moving the chatted function out of the loop -- it's usually better to avoid stacking things more than a few levels deep.

function adminSaid(msg, who)
    if msg == "Holo Open" then
        open()
    end
end

game.Players.PlayerAdded:connect(function(nP)
    for _, v in pairs(admin) do
        if nP.Name == v then
            nP.Chatted:connect(function(msg)
                adminSaid(msg, nP)
            end)
        end
    end
end)

This'll make it a little cleaner to add more commands, too, since you won't be editing the body of a connection in an if in a for in a connection.

Your problem is that you're trying to check if they're on the admin list before you define the Chatted Event. The Event fires no matter what, you can't say 'if [condition] then [event]'

Also, you have to call the function by doing; open() .

So, to fix?

local Door = script.Parent
local admins = {"mkjp87","Goulstem"}

function open() 
    Door.CanCollide = false 
    Door.Transparency = 1 
end

game.Players.PlayerAdded:connect(function(nP) 
    nP.Chatted:connect(function(msg) 
        for i,v in pairs(admins) do
            if nP.Name == v then
                if msg:lower() == "holo open" then --So it's not case sensitive
                    open()
                end
            end
        end
    end)
end)