Gitlab test webhook always got 500

I use docker to host gitlab and jenkins at different servers.
I want to trigger jenkins to run build job when gitlab pipeline is finish.
I read many articles and use gitlab plugin add jenkins webhooks at gitlab.
But everytime I click test webhooks with pipeline event. I always reload page and got 500 page. I don’t why.
Does someone know how to fix this?

To know what is the problem you would have to give more information. What do the logs say on the server receiving the webhook say?

I click the option and got another error.

Check the user name or token that you are using to authenticate.