How can one compensate for not going on Umrah due to the unavailability of a mahram?

As long as you have performed the obligatory 'Umrah (Umrah al-Islam), it is not permissible for you to travel for 'Umrah again except with a mahram. It is best to adhere to the opinion of the majority of scholars in this matter. You will be rewarded for your sincere intention, for the Prophet, peace and blessings be upon him, said to his Companions upon his return from Tabuk: "Indeed, there are people in Medina who were with you in every journey you undertook, every expense you incurred, and every valley you crossed, even though they remained in Medina, detained by a valid excuse." The doors of goodness are many; so, increase in supererogatory prayers, supererogatory charity, supererogatory fasting, remembrance of Allah Almighty, reciting the Quran, memorizing it, teaching it, and other acts of righteousness. Strive in supplication that Allah may facilitate for you the performance of 'Umrah.