Hi,
A slight dent in the penis on the backside is absolutely normal and thus nothing much to be worried about the same. However, if you’re having a problem with having an erection or sustaining it, then it’s definitely a concern. I suggest you to consult a physician and seek treatment based on the underlying cause for the same. The commonest causes to be ruled out are hypertension, diabetes, any neurological problem etc. I sincerely hope that helps. Take care.