Hi @chiara!
Since your data is still multiplexed, the manifest format is not applicable to you (those are only for demultiplexed reads).
Sounds like you have a perfect use case for importing your multiplexed reads and demultiplexing using the q2-cutadapt plugin! Check out this community tutorial for some more information:
Demultiplexing and Trimming Adapters from Reads with q2-cutadapt
Take some time to look at that document, and if you get stuck, come find us here for help!
Good luck!