https://www.cnet.com/news/spacex-aborts-first-block-5-falcon-9-launch/

SpaceX got very close to launching the final "Block 5" version of its Falcon 9 rocket for the first time Thursday, but the mission is now delayed until Friday afternoon at the earliest. 

Just about a minute before scheduled liftoff at 5:47 p.m. ET, the launch computer aborted blastoff shortly after it entered its startup mode. We're still waiting to find out the cause, but such an error is a safeguard and not that unusual.