In the Jenkin's documentation for setting up a Freestyle job there is a recommendation:

1. In the Advanced settings, set Name to origin and Refspec to +refs/heads/:refs/remotes/origin/ +refs/merge-requests//head:refs/remotes/origin/merge-requests/

https://plugins.jenkins.io/gitlab-plugin/#plugin-content-git-configuration

However setting up

+refs/heads/:refs/remotes/origin/ +refs/merge-requests//head:refs/remotes/origin/merge-requests/

into the dedicated field triggers the error (see the screen-shot attached).
The format seem to respect the documentation.