I want to make Umrah 20 days after Ramadan and I heard if I make Umrah after Ramadan Hajj becomes obligatory and I can't make Hajj this year, so can I make Umrah after Ramadan without performing Hajj?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is not compulsory on the person who performed ‘Umrah during the months of Hajj (Shawwaal, Thul-Qi’dah and Thul-Hijjah) to perform Hajj too. So, what you were told is not correct.

Allah knows best.