Difficulty with SendInBlue

I have been trying to set this up for SendinBlue with API stored in wpconfig but I keep getting a ‘Unauthorized – Key not found’ message when attempting to send (it does save OK though which indicates it is reading the define code in wp-config – I note it won’t save if it isn’t there). I have tested the same API Key with db save and it works flawlessly, just having issues with this method. What am I doing wrong?