@angular/[email protected] should be published under the latest tag, not next. See https://yarnpkg.com/package/@angular/material-moment-adapter:

FYI to install manually:
# Yarn
yarn add @angular/material-moment-adapter@^9.0.0
# Npm
npm i @angular/material-moment-adapter@^9.0.0
Fixed, thanks!
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Most helpful comment
FYI to install manually: