We recommend editing posts with working sources.
It looks that the source doesn't work because you used incorrect source. You posted thumbnail URL of photo and not actual source of photo.
It can be seen here:
I recommend adding correct source URL such as website not direct link to image (such as an example of source in steemcleaners comment above).
I will remove the flag but, please, add the correct source in the future.
RE: Lake Toba, North Sumatra (Indonesia)