Where did you get the wildflower seeds? Did it have a list of possible plants? That might help a lot, to know what the choices are.
Crush a leaf and see if it has a distinctive smell. That might be really helpful.
To me the shape of the leaf and the red stem says some variety of pigweed. The leaves look a little pale for the varieties I'm used to but maybe a different variety? It's possible it will have some flower you want but I suggest you look up pigweed to see what the seed heads look like. If it is pigweed get rid if it before it goes to seed. It makes a lot of seeds and can be invasive. You don't want it to get established. The variety I had in northwest Arkansas even had thorns, it was painful to pull.
Let us know what you find out.
