Sending SMS Error : Invalid or missing integration API Key
Zeak, 

As per Clickatell api documentation the api url to send sms is 
  https://platform.clickatell.com/messages/http/send?apiKey=xxxxxxxxxxxxxxxx==&amp;to=xxxxxxxxxxx&amp;content=Test+message+text

Please cross check in clickatell the api key under SMS Integration tab(it should be of 24 chars with '==' at the end).