I didn't test the code, but there are at least 3 (4?) errors I think.
1) missing a bracket } before "alias"
2) there is a typo in the variable name %output in the "alias handle_output", it's called %ouput
3) $bvar(&url,1).text will return ONLY the first character I think, maybe something $bvar(&url,1-300).text will fix it

and one last question, why use 2 $urlget?
I mean, if you copy a code from someone else, be more careful. Where do you get it?

Maybe this should work, I didn't try it and I don't want to.
  if ((%flood_uptime) || ($($+(%,flood_uptime_,$nick),2))) { return }
  set -eu0 %flood_uptime On
  set -eu0 %flood_uptime_ $+ $nick On

  if ($regex($msgtags(display-name).key, /^[a-z\d_]+$/ig)) { set %output msg #CHANNELNAME /me @ $+ $msgtags(display-name).key }
  else { set %output msg #CHANNELNAME /me @ $+ $nick }

  noop $urlget(https://decapi.me/twitch/uptime?channel=monstercat,gb,&url,handle_output)
alias handle_output {
  if ($bvar(&url,1-300).text) { %output channel is online for $v1 }
  else { %output channel if offline }