All sin leads to death, of course. However, Jesus' ransom sacrifice can save us from sin and death. (Romans 5:12; 6:23) Thus, a “sin that leads to death” would not be covered by Christ’s ransom. Here is the meaning of what John meant in a nutshell.
Updated American Standard Version
Giving Bible readers what God said not what translators think God meant
All sin leads to death, of course. However, Jesus' ransom sacrifice can save us from sin and death. (Romans 5:12; 6:23) Thus, a “sin that leads to death” would not be covered by Christ’s ransom. Here is the meaning of what John meant in a nutshell.