J’ai seulement une semaine de vacances

With seulement in one word means “I only have one week of vacation”.

Je n’ai que deux semaines de vacances.

Means “I only have two weeks of vacation”.

Je n’ai qu’une semaine de vacances.

Means “I only have one week of vacation” so it is equivalent to the first sentence.

It seems that there is an error in “Assimil New French with Ease”.