Here "generation" refers to the people in it. Alternate translation: "The people living at this time are evil people. They seek ... to them" or "You people living at this time are evil people. You seek ... to you"
The information about what kind of sign it seeks can be made explicit. Alternate translation: "It wants me to perform a miracle as proof that I have come from God" (See: [[rc://en/ta/man/translate/figs-explicit]])
# no sign will be given to it except the sign of Jonah
This double negative emphasizes that the sign of Jonah is the only sign that will be given. Alternate translation: "The only sign that will be given to it is the sign of Jonah" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])