There are several facts of criminal nature that may be an obstacle to the granting of a residence permit in Portugal; the practice of any fact that, if known to the competent authorities, would prevent the granting of a visa, or the indication in the Schengen Information System, or SEF's Integrated Information System, for the purposes of non-admission, such as in cases where there is strong evidence of having committed serious punishable acts, or the mere intention to commit them. In the case of a minor felony, the law requires the absence of conviction for a crime that is punishable, in Portugal, by a prison conviction of more than one year. The information provided is insufficient to pronounce in this particular case; anyway, assuming that the crime was committed 21 years ago, and also assuming that the person has lived in Tunisia for more than a year, where they have never committed any crime, I do not foresee any obstacle to the granting of a residence permit, as long as the other legal requirements are met.
