As Space Guy said; your key is valid, but it cannot connect to the Blockland Master Server.
So helpful! Space Guy's post wasn't that long that it needed a summary.
Either A: You aren't connected to the internet (Please slap yourself if you aren't.)
B: Blockland for some reason cannot authenticate or verify your DNS in order to authenticate with the Master Server, because of this it thinks that you aren't connected to the internet and goes in offline mode.
Try not to use technical words, it makes you look very stupid.