LTVP married singaporean

"She" (i.e. the person you are asking about) cannot have an LTVP and a work permit (WP) at the same time. Your post is unclear about what she has right now.
WP holders (current and former) have to seek MoM approval before they can marry a Singaporean. Failure to do so will cause great problems, including not being able to enter Singapore for many years. After marrying with MoM's approval, the WP remains valid and she can continue working.
LTVP holders do not need approval before marrying a Singaporean. However, they cannot work before or after marrying, unless they upgrade to an LTVP+ (after three years of marriage or having a Singaporean child).

Thank you for your answer

She is holding LTVP ( her child studying in singapore, n they are from indonesia ) after one year LTVP she look for job n get a work permit .
They call it special case.

Is there any difficulty to married singaporean ?

So she has a WP now, no more LTVP (she cannot have both).
As I said, as a current or former WP holder she needs MoM approval first should NOT marry a Singaporean without that, as it would cause big problems.