Health insurance has a strict legal buying window called Open Enrollment. Usually, this happens from November 1st to January 15th every year.
Why is there a deadline?
If people could buy insurance whenever, everyone would just wait until they got cancer or broke a leg to buy it, which would bankrupt the whole system. The deadline forces you to buy it while you are healthy.
What if I missed it?
If it is February and you do not have insurance, you are mostly locked out until next year. The ONLY way to get insurance is if you trigger a Special Enrollment Period (like losing a job, moving to a new state, having a baby, or getting married). Otherwise, your only backup is risky short-term insurance.