This is because Blynk uses a number of different cloud servers around the world, to get good response times.
Your project and it’s associated Auth code are only stored on one of those servers, and this will be the one that’s nearest to you when you create the project (I think it’s actually a bit more complex than that, but go with this as the explanation for now).
When you use postman on your local PC, and specify blynk-cloud.com it hits the correct Blynk cloud server, so it works.
Assume that the ITTTT servers are on the other side of the world from you, and when they send a command to blynk-cloud.com they hit a different server to the one that your postman command hits. The blynk-cloud.com server that the IFTTT server hits doesn’t have your project on it, so you get an error.
The solution is for you to open a command window on your PC and type
The IP address that’s returned is the IP of the Blynk server where your project lives. I you put this IP in your IFTTT recipe, rather than using blynk-cloud.com then IFTTT will be able to hit the correct Blynk server and will work correctly (assuming that the rest of your IFTTT recipe is correct of course).