Hi Marco,
When I build JSON for a request, I prefer to use an Export Mapping which will place the JSON in a string variable that is then available to be placed in the request body. This will save you from the need to escape brackets.
Looks like your header value is not wrapped in single quotes. The value can be a constant, variable or string however the string needs to be wrapped in single quotes. For example:
'application/json'
Hope this helps,
Sam