Foundation has a built-in checker to ensure that your stacks and theme versions are kept in sync. We have entered a conundrum with Foundation 6.12 since it now requires Stacks 5.1.
RapidWeaver does not know about that the Foundation stacks require v6.12.0. Therefore, it allows you to update the theme to v6.12. However, if you are not running Stacks 5.1, then you will not be able to update to the Foundation 6.12 stacks because Stacks knows that it will not work on your system.
I assume you are not running Stacks 5.1. You have 2 options:
Upgrade to Stacks 5.1. This may be a paid upgrade if you are running Stacks 4 or earlier.
You can downgrade the Foundation theme to v6.11 (download link). Then you need to make sure that you do not update the theme via RW in the future or else your versions will be out of sync.