Can a woman perform Umrah without a mahram?

According to the predominant view of the majority of scholars (the four madhahib), a woman is required to travel for Umrah with a mahram. This is based on the hadith prohibiting a woman from travelling the distance of three days except with a mahram. Some contemporary scholars, such as the Saudi Grand Mufti, have permitted a woman to perform Umrah without a mahram if she is travelling in a safe, organised group of women with secure arrangements. If a mahram is genuinely not available, I recommend joining a reliable, supervised group and consulting your local scholar. Allah knows best.