You can use paid commercial software to produce your solution as long as your submitted solution can be fully reviewed and used by individuals without access to this commercial software. As stated in the participation agreement, a goal of the C3.ai COVID-19 Grand Challenge is to encourage the development of contributions which may be freely adopted by the community and deployed to the areas of greatest need. In order for your submission to be useful to the community, individuals must be able to use and review your solution without access (including access via free trial) to the commercial software that you used to produce the solution.
Additionally, your submission may be made freely available in the public domain via non-exclusive royalty free (NERF) license agreements. Therefore, your submission must not breach the terms and conditions of any embedded software or services used by your submission, and must only contain materials for which you have sufficient rights to license the submission via non-exclusive royalty free (NERF) license agreements.
See the participation agreement for additional details.