I don’t think anyone is saying it’s “mandatory” for you to volunteer at your parish. Most good Catholics who are rooted in a parish do end up naturally finding some way they can help or meet a need.
I’ve had times in my life when I did some volunteer work for the parish church, Catholic school or local charity, with varying levels of satisfaction in the outcome. (Not every volunteer program is well organized or managed, and sometimes the people in charge of running it have an agenda or other issues that make it hard to work with them.) Right now I’m in a position where it would be very difficult for me to sign up as a volunteer for anything due to my schedule and other demands on my time, and I expect this situation to continue for some years. I don’t feel like I’m violating any Church rule by not volunteering if I can’t reliably serve.
If you are in a position where you think you can give a little time to a need the Church has, then be cognizant and do it. It could be as simple as getting asked to help move some chairs or stuff some bulletins at the church after Mass (one of the parishes I attend often asks the daily Mass goers to help with these chores). If you aren’t in that position for whatever reason, don’t sweat it. It’s not like everybody is expected to give 2 hours a week. One person who’s retired might have 20 hours to give while somebody else who has a job, family responsibilities, an illness etc might give 0.