That means the response of the request isn’t in JSON format. Try printing the response to see what its saying. Doing the following works. So the issue is that the request isn’t returning correctly. When I attempt to run your script I get error ‘401’ which means unauthorized. It is possible that Trello doesn’t allow for requests from Roblox.
game:GetService('HttpService'):JSONDecode('[{"id":"5ae7194d8a0d67dbb4fc252d","name":"Info","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":32767.5,"subscribed":null,"softLimit":null},{"id":"5ae714312a00d93245c4d60f","name":"Flight Number","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":65535,"subscribed":null,"softLimit":null},{"id":"5b07726c9a52dac76ac0094e","name":"Gate","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":98303,"subscribed":null,"softLimit":null},{"id":"5ae718ebb8bc7e241f0a5a79","name":"Destination","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":131071,"subscribed":null,"softLimit":null},{"id":"5ae718f8fb5e5bc812f44681","name":"Aircraft","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":196607,"subscribed":null,"softLimit":null},{"id":"5ae718fb2fe57aac26ed6526","name":"Miles","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":262143,"subscribed":null,"softLimit":null},{"id":"5ae71a22fb88f9c266aea94d","name":"Host","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":327679,"subscribed":null,"softLimit":null},{"id":"5b4b04f0bbff43dfc7192d13","name":"Airport","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":393215,"subscribed":null,"softLimit":null},{"id":"5b4b05ebd509d51b19ff4079","name":"Check-In Time","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":458751,"subscribed":null,"softLimit":null},{"id":"5b4b05eedb66bf2d14af7244","name":"Boarding Time","closed":false,"idBoard":"5ae714245c089088b8721fce","pos":524287,"subscribed":null,"softLimit":null}]')