You could, at some cost to the health of astronauts. They won't die, right away, anyway. They will lose a lot of bone density, at a minimum. The other effects, like pooling of various bodily fluids, etc. will become both tiresome and more dangerous the longer they go on. They will get a lot of radiation. On ISS astronauts work out 2.5 hours per Earth day to control health problems.

Training enough astronauts for frequent missions will be extremely difficult, maybe infeasible. Colonizing Mars is a fantasy.