Not a legal HTTP header name

I don't understand why I'm getting a message of " * message:"Content type is not a legal HTTP header name"
I know I can skip the header in this situation and it will return as a json file anyway but am really curious as to why the header isn't working. I have tried with and without the single ' marks.
the notepad file runs just fine in Quickbooks Sandbox explorer and I believe I'm setting up the header identical in Retool.

Header should be Content-Type not Content Type. The space in your header name is probably causing that.