This is because the legal responsibility lies with the host organisation in a Member State or Associated Country (future beneficiary) and only the supervisor, appointed by this organisation, can act on its behalf until the signature of the Grant Agreement. As such, only the supervisor should press the submit button and take any other actions in the name of the organisation (e.g. initiate the “review process” that for Horizon 2020 replaces the redress procedure) until the Grant Agreement is signed.
The electronic submission system will still however allow the researcher to submit the proposal. He/she should consider the implications of this carefully before doing so.
Source: FAQ Participant Portal - European Commission