Help with SIM800l module

Hello, I need help with the SIM800l module. I have connected it to an Arduino UNO with voltage regulator at 3.7 v. The led of the module turns on, but when I write AT commands, they all give me ERROR, except AT only that it returns OK. Can someone guide me? I do not understand the why of the errors in the AT commands. Thank you very much.

Debug without the Arduino Uno.

Connect directly to the module with USB-TTL adapter to communicate using AT commands.

Make sure you can do what you want before writing code.