Under the CARES Act, if you are claimed, or qualify to be claimed, as a dependent on someone else’s 2019 return you cannot receive a stimulus check, in 2020. If you qualified as a dependent for 2019, but will not be for 2020, you will most likely get it in 2021, when you file a 2020 tax return.
Note that the requirement is not just whether you are actually claimed as a dependent, it's whether you qualify to be claimed as a dependent.
