Why can't I tweet? What is "follow of"?

Hello. I suspect you're getting that message because you had tried to send a tweet that starts with "d of ...."

I say this because 'd' is a special keyword in Twitter that stands for 'direct message.'

It's used in this way: "d username your message"
For example: "d evan how are you doing?"

Twitter only allows us to send direct messages to people who are following us. If we try to send direct messages to someone who's not following us, Twitter returns a message similar to what you saw.

For example, the user 'cnnbrk' does not follow me. When I type
'd cnnbrk how are things?'

... I get a reply that reads
"cnnbrk does not follow you. Send 'follow cnnbrk' to request."

Since you got a message that says "of does not follow you. Send 'follow of' to request," it's my guess that you had tried sending a tweet that starts with 'd of' -- Twitter thinks you're trying to send a direct message to a user named 'of'