Encrypting a pipeline for elastic encoder

I have been trying to setup encryption for a pipeline for a while now and i always get this error.

This Pipeline does not have an AWS master key assigned to it. Assign a master key to the pipeline to use this mode. Learn more

Do i need to set this key and how?

AWS KMS Key ARN

If i can it to custom and try and paste my key in i get a error.

Where is the documentation on how to do this please help.

Thank