What is the ERC (European Research Council)? Find more information here:

-ERC website
-Euresearch: the Swiss guide to European research & innovation

Q: Can a Starting, Consolidator or Advanced Grant holder apply for a Synergy Grant?

The Work Programme 2018 says: A Principal Investigator who holds an ERC frontier research grant cannot submit a proposal for another ERC Grant unless the existing grant expires no more than two years after the call deadline. This means that there should be two years or less between the call deadline and the end of the ERC project.