Cache Site Pages on Multisite?

Hi, when trying the beta Page cache on a multisite installation but keep getting the message:

The configuration file cannot be updated

The configuration file, wp-config.php, is not writable. This file must be writable to activate caching. For instructions on how to make this change, please click here.

The file is writable…

I can imagine the feature is not compatible with multisite. Is that why it is failing?