Roger Federer underwent two knee surgeries in 2020

Federer underwent his first knee injury after the 2020 Australian Open, which ended in early February. He intended to return to action at Wimbledon in June, but the COVID-19 pandemic and knee complications prevented that.

Source : https://www.washingtonpost.com/sports/2021/03/26/roger-federer-knee-comeback/

Roger Federer try to make a comeback after his knee surgeries but his body seem need more recovery attention before he can put 100% effort into his game . To be in game after surgeries is very hard , and the time of rehabilitation is not even reach 1 year , was too much for him to handle. The decision to withdraws from Frence Open is a right decision for his long term carrier.