There is no problem of making supplications after an obligatory prayer. It is preferable to pronounce your du`aas before tasleem and after reciting tashahhud. It was narrated from `Amr ibn Shu`aib, from his father, that his grandfather said: “I heard the Messenger of Allah (peace be upon him) say: “Allahumma inni a`udhu bika minal-kasali, walharami, wal-maghrami, wal-ma’thami, wa a`udhu bika min sharril-masihid-dajjali, wa a`udhu bika min `adhabil-qabri, wa a`udhu bika min `adhabin-nar” (O Allah, I seek refuge in You from laziness, old age, debt and sin, and I seek refuge in You from the evil of the Anti-Christ, and I seek refuge in You from the torment of the grave, and I seek refuge in You from the punishment of the Fire.)'” (An-Nasa’i)

 

But if you offer your du`aa’ after prayer and finishing dhikr, there is no problem. `Ali ibn Abi Talib said: When the Prophet (peace be upon him) uttered salutation at the end of the prayer, he used to say: “O Allah, forgive me my former and latter sins, what I have kept secret and what I have done openly, and what I have done extravagance; and what You know better than I do. You are the Advancer, the Delayer, there is no god but You.” (An-Nasa’i)

 

Moreover, Mu`adh bin Jabal (may Allah be pleased with him) reported:

Messenger of Allah (peace be upon him) held my hand and said, “O Mu`adh, By Allah, I love you and advise you not to miss supplicating after every Salat (prayer) saying: ‘Allahumma a`inni `ala dhikrika wa shukrika, wa husni `ibadatika,’ (O Allah, help me remember You, expressing gratitude to You and worship You in the best manner)”. (Abu Dawud)

 

Almighty Allah knows best.