Learn Spanish (Latin America): Él todavía no ha comprado el cartón.

Todavía means "still" or "yet." When you want to say someone "has not yet" done something, you use todavía no. In Spanish, you need the no to make the sentence negative, so it translates closely to "He still has not bought the cardboard."

Can I change the order and put todavía at the end of the sentence?

Yes! You can say Él no ha comprado el cartón todavía. Both are perfectly natural. Just remember that the no must stay right before the verb (ha), while todavía can go at the beginning or the end.

Why can't I put todavía no between ha and comprado, similar to English?

In Spanish, the two parts of the present perfect (ha and comprado) can never be separated. Extra words like todavía no, ya, or siempre must go before the helping verb (ha) or after the whole verb phrase, never in the middle.

Does cartón mean "carton", like a carton of milk?

It can, but its primary meaning is the material "cardboard." To say "a carton of milk," you will often hear una caja de leche (a box of milk) or un envase, though un cartón de leche is used in some regions too.