Find sessions return emtpy,Join Sessions Error

Hi,I have a question to ask.I’am using Unreal Engine 4.18.3.!In this buleprint I have 3 buttons,One is Create,One is Find and one is Join.When I press the button in server,It says Created.And then I clicked the Find Button,it says Find Ok.But when I clicked the Join Button,It says Join Error.I think the result of the find sessions is empty.Why would it Be empty?And Why I can’t join the sessions but some of my friends can do it in their PC?Please help me ,I hurry.Thanks

[alt text][2]