Here is the keypad addon I use and I can confirm this method works on: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=108424005
What you will need:
No Lua Experience, just follow what I say.
Notepad++

Alright firstly if you want to fix this on your server your going to want to close your server down completely.

Navigate to your servers addons folder.
Open up your Keypad's addon folder.
Navigate Lua\autorun\client
Open keypad.lua with Notepad++

Now were going to want to enable gmods lua format onto your notepad++, goto plugins on the top of notepad++, open plugin manager, scroll down the availible until you find gmod lua lexer and install it, once its installed you can close the plugin manager. (This is to ensure the color coding is correct, since I will be using colors to identify what you need to change)

Alright Control+F and type in keypad, find next, basically any GRAY (not black) words that say Keypad, your going to want to make them un captialized. So if you see a gray "Keypad" change it to "keypad".

Once every gray keypad is lowercased, save keypad.lua and close.
Go back to your lua folder in keypad and go to entities.
Open the keypad folder and do the exact same thing you did to keypad.lua to EVERY other lua file.
Navigate back to entities and goto keypad_wire, do the same thing. (Turn "Keypad_Wire" into "keypad_wire"

Thats it, I know its alot of work but this addon uses the lowercased keypad classname and most keypad addons use the uppercased classname.