>>3078732 (LB)

The last verses in Mark 16 verses 15 and beyond to the end of the chapter, where you find those references, were added later after the Gospel of Mark was written.

Any good study bible will point that out.

John MacArthur Study Bible note on Mark 16 has that but also Ryrie Study Bible does too.

Well known where they leave it in but the best manuscripts do NOT have that section at all.

Added after 100 AD. Mark was written in 55 or so AD.