Canon (Church law) prohbits the washing of women’s feet as part of the Holy Thursday service on the basis that Jesus only washed the feet of the Apostles, who were all men. This also relates the the Church law that only men can become priests.
This rule, however, is not dogma - such as belief in the Triune God, eg. and the Pope has the right to change the Holy Thursday rule. Instead of opennly changing the law, Pope Francis just ignored it. Some people believe that this means Pope Francis is open to women priests or deacons. Others are upset b/c they think that rather than breaking the law, Pope Francis just should have changed it as it may invite other breaches of canon law.