"Mereka bisa menyelesaikan buku itu dalam tiga minggu."

Translation: They can finish that book in three weeks.

[Pak_John]

Im confused as to what the -kan adds to this sentence?

[UpzahwvW]

It's causative/benefactive, so for this sentence it is necessary. They are causing the book to become finished (possibly by reading it).

[Lalunatech]

'the book' can also be translated into 'buku itu'