"A no is a no."
Translation:Et nei er et nei.
Like in English, you need an article for a determiner (no).
"nei er et nei" = no is a no.
When you talk about the word no as a noun, it takes the neuter article.
As far as I'm aware, there are no rules - you just have to learn the gender of the noun when you learn the noun. With living things it usually follows natural gender (so man is masculine, etc), but even that's not 100%.
The article used with neuter nouns is et, e.g., et hus / huset.
The article used with masculine nouns is en, e.g., en bil / bilen.
The en article can also be used with feminine nouns, e.g., en seng / sengen. Alternatively, you can use ei as the indefinite article with feminine nouns, e.g., ei jente, and an a ending for the definite form, e.g., jenta (the girl).