Hi Patrick,
The reason this might work in the structured data testing tool is that the validation done by the Google tool ignores some of the errors in the json, probably because the Google crawler knows how to handle these.
However, the validation done in the SEO panel checks that your code is in a valid JSON format.
So, if you are still facing issues with validating your code, I recommend using a 3rd party json validator.
There are many free json validators online that can do the trick. Here is one I found with a simple Google search - https://jsonformatter.org/
If you are still facing issues saving your structured data even after using this tool, feel free to paste the code you are trying to add to this thread or open a support call and reply here with the ticket number, and I’ll be happy to take a look.
Thanks,
Adam