Please don't use the bug tracker to request help. Many more helpers are available on internet relay chat (the #jenkins channel). Refer to chat for more instructions on IRC. If IRC does not provide the answer, you can use the Jenkins users mailing list. It also appears that you can purchase Jenkins assistance from codementors.
In this specific case, you're probably not providing the credential to the git step. If you are providing the credential, then it may be an incorrect credential.
You may also want to consider using the checkout step, and use the "Pipeline Syntax" link on the left side of the job page to let it help you provide the desired arguments to the checkout step.