What should I do about not completing Umrah after assuming Ihram, and the consequent masturbation and ignorance, and how can I complete it or atone for it?

It was obligatory to complete the 'Umrah, and it is not permissible to abandon it. You must now refrain from what a pilgrim in ihram refrains from, then go to Makkah to perform theks.

Scholars differed regarding masturbation during ihram: does it invalidate the ritual or not? The majority opinion is that it does not invalidate the ritual, and requires a sheep. The Hanbalis, however, hold that it requires a badanah (camel or cow).

There is no excuse for ignorance in this situation. This type of prohibition is akin to destructive acts, where intent and ignorance are equally blameworthy. If the fidyah (expiation) is a sheep, then it offers the same options as the fidyah for harm: fasting three days, or giving three sa' of food to six needy persons.