The National Science Foundation will now start requiring the use of Research.gov for the preparation and submission of proposals in response to many (but not yet all) program descriptions.
Follow the steps in the notification on Research.gov to determine if your proposal for a specific program description must be prepared and submitted in Research.gov.
Note that FastLane is targeted to be removed as a submission option from all funding opportunities when the Proposal and Award Policies and Procedures Guide (PAPPG) goes into effect in January 2023. Grants.gov will remain a submission option for most NSF proposals.