The following extract is taken from a book published by Oxford University Press (I have attached the screenshot of this part of the book. You can have a look at it if you like)
Everywhere I looked, I saw couples holding hands and looking romantically into each other's eyes! The travel agent also said it was always sunny there - but the weather was terrible - it was cloudy and windy. To pass the time, I decided to take a diving course (one of my lifetime ambitions). But it was a complete disaster because the first time I went under the water, I had a panic attack. I couldn't escape from the island, so I spent an incredibly boring (and expensive) two weeks in "paradise.
I don't understand why 'an' and 'two' can be used concurrently in this sentence. Is it grammatically correct? Should it be written as so I spent two incredibly boring (and expensive) weeks in 'paradise'
Thank you very much