Art. 75 of the Family Code provides that the future spouses may, in the marriage settlements, agree upon the regime of absolute community, conjugal partnership of gains, complete separation of property, or any other regime and in the absence of a marriage settlement, or when the regime agreed upon is void, the system of absolute community of property as established in this Code shall govern.

My question is, when can you say that the regime agreed upon is void?

Thank you.

The regime agreed upon is void when the marriage settlement (pre-nuptial agreement) was done after the marriage, since it is invalid the default regime would be Absolute Community of Property not the regime agreed upon in the settlement.

can the couple living together without the benefit of marriage can make an arrangement regarding their property? if so, is it valid? what law will govern their agreement?

is living together without the benefit of marriage a licit or an illicit relationship?

Couples who are not married CANNOT make any arrangement regarding their property if you're pertaining to a "marriage settlement" type of arrangement, if there is any then it is INVALID. However the law has a specific property regime for unions that does not have the benefit of a marriage, Art. 147 and 148 (adulterous relationship) of the family code (Property Regime of Unions without marriage) applies.

It depends. If both unmarried couples are single meaning both are capacitated to marry each other without any legal impediment then it is not illicit, in fact there is no need of a marriage license when the unmarried spouses decide to get married provided that they have been cohabiting for at least 5 years.

Art. 34 states:
"No license shall be necessary for the marriage of a man and woman who have lived together as husband and wife for at least five years and without any legal impediment to marry each other...XXXXXXXXX"

The law even encourages unmarried couples to get married and makes it easy for them.

However, if either spouses has an existing valid marriage then definitely their relationship is illicit. They could be either criminally liable for concubinage or adultery and in case they get married, for bigamy.