Make sure that you are in the correct mode. Enabled and in configuration mode. The prompt should look like:
Also, a while back IPv6 was added to the configuration of most AOS versions. This slightly changed the syntax. Many layer 3 commands require "ip" before the command, or "ipv6" if you're configuring ipv6 protocol.
Also, there are some commands that are specific to a section of the configuration and others that are global. If you're configuring an interface, global commands won't be recognized.
You can see the available commands by typing a "?" at any point.