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MoM does not normally deal with job seekers directly. If you want to visit them, let your employer arrange a meeting with the officer there - and they can take you along.

I've tried applying to many companies online but they seem not to reply back and they said at the last stage of applying that my application is successful but not email from to send me employment approval or work letter. Can someone tell me what may have caused it

Salle Victorine: It is of course difficult to say what went wrong in your specific case, based on the very little information you gave.
In general, Singapore companies do not send out negative replies. You only hear from them if you're shortlisted for the next stage (typicall one or more interviews).
Also, Singapore employers do not pay for travel to an interview. For that reason applicants who are outside of the country are often thrown out. Against that, you could arrange a job search trip (on your own cost) and mention in your application "I will be in Singapore from AAA to BBB and that would be a good opportunity to meet you in person."
(Do not expect, though, to get a job offer or even receive an approved work visa during such a visit  - things will take a few months!)
And, last but not least, your application must be very targeted to jobs needing exactly your proven skills (you should explain in your cover letter why you are better suited than other potential applicants - and much better than local ones, who must be preferentially hired). It helps, in this respect, if your skills and prior work experience (of which you need three years or more to have a realistic chance) are in an area with manpower shortages in Singapore - so do your own research first!
And another sad fact is that many Asians are prejudiced against Africans (I guess from your origin country Cameronn that you are Black?). Your chances are higher at American or European companies.