It appears the hash in the formula is incorrect for the 1.1.54.0 bundle.
% brew install aws-session-manager-plugin
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/cask).
No changes to formulae.
==> Installing aws-session-manager-plugin from dkanejs/aws-session-manager-plugin
==> Downloading https://s3.amazonaws.com/session-manager-downloads/plugin/1.1.54.0/mac/sessionmanager-bundle.zip
######################################################################## 100.0%
Error: An exception occurred within a child process:
ChecksumMismatchError: SHA256 mismatch
Expected: 57f310fa55944fe8d894d7d03b2b0b1f657a2d33b740a9a248dd15f47183fddf
Actual: d9b558193370b2ecc0ddba001b6ee974b14b60d4d247851706e26a9811f15349
Archive: /Users/catmeme/Library/Caches/Homebrew/downloads/a419a4afbd34b9a4edf572409ad606c2b7d776923946c9f9d78a1d3560618a74--sessionmanager-bundle.zip
To retry an incomplete download, remove the file above.
I have manually downloaded the zip, and calculated the sha256 sum, receiving the same result as brew:
% curl -O https://s3.amazonaws.com/session-manager-downloads/plugin/1.1.54.0/mac/sessionmanager-bundle.zip
% shasum -a 256 sessionmanager-bundle.zip
d9b558193370b2ecc0ddba001b6ee974b14b60d4d247851706e26a9811f15349 sessionmanager-bundle.zip