from the yt description:
A tribute album to the world famous producer David Foster, [HITMAN PROJECT] releases the second album "Hard To Say I'm Sorry".
The second album of [HITMAN PROJECT] is one the Korea's favorite pop songs, ranked 8th "Hard To Say I'm Sorry" sung by the legendary pop band Chicago. For this album, music director Lee Seunghwan, best Korean female vocalist Lee Sora, and YoungJi worked together.
Producer Lee Seunghwan directed the OST of A Love Story(Siwolae), Love Letter as well as Lee Sora's 'The Wind is Blowing', 'Dancing with You', and 'Not Anymore'.
Lee Sora's voice is amazing♥ You might remember her from her feature in Tablo's Home (though she's known for a lot more than that). I honestly didn't watch the video, but the song sounds amazing.