hello i have problems my friends doesnt have op and they cant buy any item how i can repair it

This is usually caused by a plugin interfering with SignShop or the built in vanilla spawn protection. Try changing spawn-protection=16 to spawn-protection=0 in server.properties. If you have Essentials installed try disabling all of the Essentials signs including -color by commenting them out. Otherwise you can use your permission plugin's verbose logging to see what permissions are being checked when they try to use a sign. i.e. /LuckPerms verbose on or /pex toggle debug.