Why is that you cant still penetrate your partner?

It is assumed that you are a male and a female.
You must prepare the vagina for sex with foreplay and stimulation with a finger until it is slippery or you must add lubrication gel to make it slippery. If the vagina is very small and cannot admit a finger there is a medical problem or your partner is a virgin.
If she is a virgin she may have an intact hymen (maidenhead) which can usually be broken by finger or penis pressure. If the hymen is too tough to break, it is better to have it medically cut open.