There’s no cancer vaccine because cancer isn’t a virus. It comes from our own cells’ genome running amok and causing the cells to divide uncontrollably. For the other viruses, mRNA might work, but there isn’t a single characteristic protein that we can encode into an mRNA molecule that will stimulate the immune system like the spike protein in the case of the coronavirus that causes COVID.
It’s also not easy to isolate particular sequences in a genome, even a relatively small one like a viral genome, that encode particular proteins. We actually had a head start in this case; work along these lines was done on past related viruses that caused lesser epidemics, SARS and MERS for example. The work wasn’t completed in these cases because those epidemics died off before vaccines could be developed. But those efforts laid the groundwork for the current vaccine.
Obviously.
But, a cancer vaccine can certainly alert the immune system to attack tumors, or attack specific molecules inside tumors.
I wrote my original comment because you stated that mRNA vaccines are safe because we know almost "everything there is to know about mRNA."
If that is true, it seems to me that mRNA cancer vaccines would be common and effective, and clearly they are not.