Peru's captain and striker and national hero Guerrero was banned due to cocaine use, which he says was accidental. His ban was then reduced to six months meaning he could play. CAS appealed and his ban was increased again to 14 months, meaning he misses the tournament.
A few days ago Mile Jedinak, Hugo Lloris and Simon Kjaer - captains of the countries in Peru's group C - wrote to Fifa asking that he be allowed to play. Guerrero even met with Gianni Infantino, but at the moment it seems like he won't be playing. That appears to be a very sporting gesture on behalf of the other captains.
https://www.theguardian.com/football/2018/may/22/rival-captains-make-case-to-fifa-for-paolo-guerreros-world-cup-inclusion