AWS IAM user receive 401 when accessing to ECR repository works with root user

Question

I've started using AWS ECR to store my docker images. When I try to authenticate an IAM user, via Powershell (the same happens when I do via AWS command line) I receive a 401:UnAuthorized. If I use the Auth key/secret of the root user, it works and authenticates.

The PowerShell script I use is

(Get-ECRLoginCommand).Password | docker login --username AWS --password-stdin 474389077978.dkr.ecr.eu-west-3.amazonaws.com/myreoi

I've replaced the AWS user with the IAM user. I've also added the IAM user to the admins, but it doesn't seem enough.

Any suggestion? Thanks in advance!