This google translated to
Reviewers gave these main reasons for not accepting this proposal:
Access to the proposal may not be secure or publicly accessible
In that supporting photo, I was also wondering if this sign had safe pedestrian access. If you do try to resubmit, you will need a supporting photo that demonstrates that pedestrian access is safe. I don’t know how to suggest that from what I see here. I am not familiar with this part of the world, so I would recommend you get some more local responses before trying again.
You could also try an appeal to Niantic if you believe that you can prove this sign is safe for pedestrians to access. But keep in mind you only have 2 appeals, each on a 20 day counter before you can use them again.
Edited to include the link to the clarification about trail markers since that was the original question. This appears very similar to the first example to me: