I’m at home
The reason for the delay was the need for additional testing of the Starliner manual control system. It is needed in case of a software failure so that the ship does not lose control.
NASA postpones first manned flight of Boeing’s Starliner
The first Starliner crew will include astronauts Barry Wilmore and Sunita Williams. If everything goes according to plan, they will have to spend about two weeks on board the ISS. After the successful return of the ship, it will receive NASA certification for regular manned flights, such as SpaceX previously received with its Crew Dragon spacecraft.
Note that the development of Starliner has been going on since 2014, but so far the ship has not received permission for regular flights.