it is a must to gain the certificate.

is "must" here informal voice or formal voice?
Hmm. 'Formal' and 'informal' are not voices-- the voices are 'active' and 'passive'-- but I know what you mean, Vincent. 'Must' as a noun has been around a long time, but I suppose if I were writing a proper 'formal' sentence, I would use 'requirement'.
Dear Vincent Ding,

I must say a "must" belongs more to a frivolous vocabulary/world. I do agree with Mister Micawber: use "requirement" in a formal context.

Kind regards,
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The noun "must" may not be as formal as "requirement", but it is much used in many serious contexts, and can hardly be characterized as "frivolous"!
Dear CalifJim,

It is true. «Frivolous» is the wrong word. Thank you for the clarification.

Kindest regards,
Tks CalifJim.
and i also find that requirement could not always take the place of MUST.

for example: In that event, it is a must to commence proceedings against the seller if the buyer intends to seek damages for his losses.

don't you think so?

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Vincent ding! I guess you made a perfect example of using "a must" instead of "requirement".
Try to disprove me if I am wrong...