The TYPO3 install tool doesn't yet fully understand when you are working on a cloud envirionment and may warn you that some folders are not writable.
Don't worry, your TYPO3 installation will be fully functional.
TYPO3 extension can easily be added using composer. Just use the platform.sh CLI tool to run composer, as follows:
platform get <project id> -e <branch name> composer require typo3-ter/[extension name] git add composer.* git commit git push