I am sorry that you are encountering this error message when trying to activate your Welcome Email. This is a known issue within our system that we are currently working on fixing. The workaround for this issue is to recreate the Welcome Email. We can recreate the Welcome Email for you, but will not be able to do so by today. You can also create a new email if this needs to be done sooner, by following these instructions.
What step did you take before you encountered this error message (ex: changed font color, inserted an image, etc.)?. We are trying to determine why this issue is occurring, so any further information would be most helpful.
If you run into any additional issues or have more questions, don't hesitate to reach out.