This error message is usually caused when you do not have scl-utils installed. Try doing

yum install scl-utils

The scl application is delivered with the scl-utils package. The scl-utils package should have been installed as a dependency of devtoolset-7-gcc.

I think I see what happened though, if you have your full output from when you ran yum install centos-release-scl scl-utils-build devtoolset-7-gcc*, you may notice a line No package devtooset-7-gcc* available.

This would be due to trying to install an additional repository and a package from that repository at the same time (the other two packages should have installed fine though). YUM does not work like that, as the new repository metadata would not be built until the next run of YUM and would not be available in the same transaction that installed the repository.

If you separate your install command into two commands, YUM should be able to install the devtoolset-7-gcc* packages:

yum install centos-release-scl scl-utils-build
yum install devtoolset-7-gcc\*

Although the @amzn-main repo doesn't have PHP 7.2 yet (as far as I know), you can use remi-php72. According to his release blog you can install the EPEL and Remi repositories via:

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-6.rpm
rpm -Uvh remi-release-6.rpm
rpm -Uvh epel-release-latest-6.noarch.rpm

And then enable remi-php72 using yum-config-manager:

yum install yum-utils
yum-config-manager --enable remi-php72

After that, you can simply search and install php and all the needed extensions like normal:

sudo yum install --disablerepo="*" --enablerepo="remi,remi-php72" php php-fpm php-cli ...