Provisioning manager has no rule of not selecting root aggr (aggr0) for provisioning, if the required conditions are met could select it.
Now for your case, Free space is not the only condition needed for provisioning, its should meet all conditions of your policy, your aggr overcommitment %, licenses, etc. If both your aggr are shown as RED crossed, try the automatic resource selection radio button at the provisioning page and go to the next step. The next step will fail and will exactly point out the reason for Provisioning Manager (or DFM) not selecting any of the 2 aggrs.
Can you paste this dry-run result( copy to clipboard and let us know)? With that the real cause can be pointed out.
If this post resolved your issue, help others by selecting ACCEPT AS SOLUTION or adding a KUDO.