I had an “interesting” experience at daily mass the other day in which a nun celebrated mass. She read the gospel, gave a homily, skipped the apostles creed, and during the Liturgy of the Eucharist, she skipped right to “Behold the Lamb of God, Behold him who takes away the sins of the world”. Is it licit for a nun to say mass?
No, a nun cannot celebrate Mass (nor can anyone who’s not a priest or bishop).
Based on your description it doesn’t sound like it was a Mass but rather a communion service. A communion service differs from Mass in that there is no offering of the sacrifice of the Eucharist. It is simply a prayer service with readings and there can be a brief reflection given by a non-ordained person if the Bishop permits it (canon 766). The Eucharist is then retrieved from the tabernacle and distributed to the faithful.
Archdiocese of Atlanta’s Order for a Communion Service by a Lay Person