Do you have EPEL enabled on your machine?
Edit:
Checked on my AMZN2 system, things seem to work ok:
google:amazon-linux-2-64 /# yum repolist
Loaded plugins: langpacks, priorities, update-motd
216 packages excluded due to repository priority protections
repo id repo name status
amzn2-core/2/x86_64 Amazon Linux 2 core repository 25526
amzn2extra-epel/2/x86_64 Amazon Extras repo for epel 1
epel/x86_64 Extra Packages for Enterprise Linux 7 - x86_64 13403+216
snapd-amzn2/x86_64 snapd packages for Amazon Linux 2 4
repolist: 38934
And a snapd from that repo is properly listed:
google:amazon-linux-2-64 /# yum list snapd\*
Loaded plugins: langpacks, priorities, update-motd
216 packages excluded due to repository priority protections
Available Packages
snapd.x86_64 2.51-1.el7 epel
snapd-debuginfo.x86_64 2.51-1.amzn2.1 snapd-amzn2
snapd-devel.noarch 2.51-1.el7 epel
snapd-glib.x86_64 1.58-1.el7 epel
snapd-glib-devel.x86_64 1.58-1.el7 epel
snapd-glib-tests.x86_64 1.58-1.el7 epel
snapd-qt.x86_64 1.58-1.el7 epel
snapd-qt-devel.x86_64 1.58-1.el7 epel
snapd-qt-qml.x86_64 1.58-1.el7 epel
snapd-qt-tests.x86_64 1.58-1.el7 epel
snapd-selinux.noarch 2.51-1.el7 epel
As mentioned before, you can install it passing the complete version information:
google:amazon-linux-2-64 /# yum install snapd-2.51-1.amzn2.1
Loaded plugins: langpacks, priorities, update-motd
216 packages excluded due to repository priority protections
Resolving Dependencies
--> Running transaction check
---> Package snapd.x86_64 0:2.51-1.amzn2.1 will be installed
--> Processing Dependency: snap-confine(x86-64) = 2.51-1.amzn2.1 for package: snapd-2.51-1.amzn2.1.x86_64
--> Running transaction check
---> Package snap-confine.x86_64 0:2.51-1.amzn2.1 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
===============================================================================================================================================================================================
Package Arch Version Repository Size
===============================================================================================================================================================================================
Installing:
snapd x86_64 2.51-1.amzn2.1 snapd-amzn2 16 M
Installing for dependencies:
snap-confine x86_64 2.51-1.amzn2.1 snapd-amzn2 2.8 M
Transaction Summary
===============================================================================================================================================================================================
Install 1 Package (+1 Dependent package)
Total download size: 18 M
Installed size: 52 M
Is this ok [y/d/N]: ^C