May a Principal Investigator (PI) whose proposal was evaluated in the ERC 2018 Synergy (SyG) call as category B or C in Step 1 or B in step 2 submit a proposal to the ERC 2019 SyG call?

No, a Principal Investigator (PI) whose proposal was evaluated as category C at step 1 or, B at step 1 or 2 in the 2018 Synergy (SyG) call may not submit a proposal to the 2019 SyG call.

Further details in the ERC Work Programme 2019, section Restrictions on submission of proposals.