i can´t change survival to creative

to change the gamemode you need the command

/gamemode 1

for creative, for survival just replace the 0 with a 1.

if this doesn't work, try to op yourself through the console (type /op [username]) and then retry it, or do the command /gamemode 1 [username] in the console

When changing settings at the My Server page it's best to do it while the server is offline.