Hi its lacrosse17.
Fix-
1)Go into your server.properties folder
2)look for
snooper-enabled=true
online-mode=true
pvp=true
3)Change it to this

snooper-enabled=true
online-mode=false
pvp=true
4)Start server
(If Started stop server then start)
5)Join server
Please Msg me if you need help
-Lacrosse17
P.s. This worked for me and my friends server
(lacrosse17.no-ip.org)Join to see it works
Oh and Diamond and sub to me if you need help with anything eles!