It is important to have well-defined structured data LD+JSON. But when the structure is malformed then it impacts the processing by search engines as they will have difficulties understanding the structure. The rule checks for whitespaces and ensures it gets parsed without issues.
How to fix it
Make sure your structured data LD+JSON is being represented as a single string, without whitespaces, especially new lines.
If you’re using WordPress plugin Markup (JSON-LD) structured in schema.org then enable (check the checkbox) option Compress output data under section Schema.org Config.
For PHP language use: json_encode($value, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES).