Looks like you may be trying to install to /Applications from an account that doesn't have admin privileges. Try installing to the Applications folder in your user directory, ~/Applications, instead.
Admin users should always be able to write to /Applications without requiring privilege escalation, no need for a password prompt in that case. Probably worth trying with the latest beta build from the downloads section, the 5.18 installer is 18 months old so might have some compatibility issue with the latest macOS versions.
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