I think I got it solved. I am not sure this is the cause. But I found it very interesting. 1. Created a user pool on us-west-1. Run the command: % aws cognito-idp list-user-pools --region us-west-1 --max-results 10 { "UserPools": [ { "Id": "us-west-1_mUijhrybU", "Name": "--", "LambdaConfig": {}, "LastModifiedDate": "2023-01-20T14:12:50.797000-08:00", "CreationDate": "2023-01-20T14:12:50.797000-08:00" } ] } 2. Run the command again with no region parameter: % aws cognito-idp list-user-pools --max-results 10 { "UserPools": [] } 3. Created a user pool on us-east-1. Run the command: % aws cognito-idp list-user-pools --max-results 10 { "UserPools": [ { "Id": "us-east-1_7nUtrepfj", "Name": "----", "LambdaConfig": {}, "LastModifiedDate": "2023-01-20T14:26:53.394000-08:00", "CreationDate": "2023-01-20T14:26:53.394000-08:00" } ] } Interesting. So when I call the cognito-idp Signup API from cognitect aws api, I got this response on the REPL: {:UserConfirmed false, :CodeDeliveryDetails {:Destination "h*@g*", :DeliveryMedium "EMAIL", :AttributeName "email"}, :UserSub "53bc5c-------------------24----d52"} Problem solved by creating the USER POOL in the US-EAST-1 region!! (A bug in the AWS CLI?)