What does this code mean?

[tiffers8433]

I received this text a few times after I used my texts after just putting in a new SIM card to get my existing phone number on a new phone as mine was recently stolen.  What exactly does this mean?

(IM404) This number is inactive or you have sent an unknown keyword.

[FidoJulien]

Hey @tiffers8433

Welcome to the community!  

What type of phone are you using?  Also, does this message prevent you from sending text messages?