Hi @Rishikesh12,
Happy to help you get your data imported! 
Just FYI I've moved your question over to user support since you are asking specifically how to import data into QIIME 2 - if you have any questions regarding PICRUSt2 functionality, I'll just ask that you create a separate post for that down the line.
With that being said - you can import a FASTA file into QIIME 2 by following particular formatting requirements that we have. Here's a great post with reference to those requirements, which also references our importing docs that discuss this as well.
Hope this helps! Cheers 