enabling disabling repos

It's also possible that yum update -y is responding like that because all the installed packages are already up-to-date. What AMI are you using, and when was the host last patched?

Don't put the exclamation mark " ! " as part of the command. The shell is expanding that and looking for the most recent command starting "amzn2-core/2/x86_64", which of course isn't in your shell history, and so the entire command is failing.

Steps to clear the exclamation mark are at https://access.redhat.com/solutions/2267871 (basically, run yum clean expire-cache). Then yum check-update to see what packages have an update available, and yum update -y to install them.

Enable a repo that is disabled with e.g. yum-config-manager --enable amzn2-core-debuginfo (only the bit of the name up until the first forward slash).