Googled this and realised DS918+ is not supported for this.
Reddit users talking about install docker DSM inside DSM to bypass the restriction and I thought if it runs on Docker then it’s not CPU arch type issue, just some config level restriction. Which turns out to be true.
To bypass it, download the docker package from below URL and get a hex editor.
Open the file and edit archtype dockerx64 to x86_64 then ensure backfill the reduced characters into places like description as it does have size check.
Manually install the package file from the package manager. It will prompt for crc check error, but you can ignore and continue.