It is important that you attend Mass each week and on Holy Days of Obligation (see the Precepts of the Catholic Church). However, as a disciple in training, you should be able to reflect on the readings, liturgical music, homily and other aspects of the Mass. Your Sunday Mass Reflection Sheet give you the opportunity. It is also a great chance to review Scripture, the liturgical calendar, parts of the Mass as well as different occasions for Mass.
You can certainly complete a Sunday Mass Reflection Sheet if you attend Mass at a different parish.
If you did not attend Mass or participate virtually, you can still fill out your Mass Reflection sheet by meditating on the readings. You can find the readings at USCCB website.