Under Article 128 of the Singapore Constitution, a citizen of Singapore can only renounce his/ her Singapore citizenship if he/ she:

a) Is aged 21 and above;
b) Is of sound mind; and
c) Is also or is about to become a citizen of another country.

The Government may withhold the renunciation of Singapore citizenship by a male citizen if he is subjected to the Enlistment Act (Cap. 93) unless he has
a) discharged his liability for full-time service under section 12 of that Act;
b) rendered at least 3 years of operationally ready national service under section 13 of that Act in lieu of such full-time service; or
c) complied with such conditions as may be determined by the Government

You may refer to Renunciation of Singapore Citizenship for more information.