Is an 'Umrah permissible if one doubts that a drop of urine was discharged after performing ablution, then prayed? And what do you advise?

He is not required to repeat ablution, nor prayer, nor wash anything from his garment as long as he is doubtful about the drop having exited. The fundamental principle is the persistence of purity until certainty of impurity is attained, in accordance with the hadith: "He should not leave until he hears a sound or perceives a smell." And his 'umrah is valid.