It is always difficult to maintain a string variable with JSON, since you need to escape all double quotes. My recommendation would be to keep the JSON template in a text file with place holders for variables. Read the text and replace the place holders with correct variable values using UiPath, and then use it as the JSON string for the api calls.
Like others I don’t recommend manually compiling your JSON in that way. loading it from another source or storing it as an object and converting it to JSON would be more manageable.
I did watch half the video and in the context you are missing your quotes when using your variable.
Good
"{""title"":""Prueba"", ""body"":""also es algo""}"
Bad
"{""title"":""Prueba"", ""body"":"+Texto+"}"
Unless the Texto variable itself contains double quotes in the string, then you need to add double quotes in your JSON template to encase it.