Hi @Raima,
Welcome in the forum!
Yep, the initial importing is usually one of the challenging one!
Although they are not strictly related to fungal analysis, I would still suggest to start working from the QIIME2 tutorials, then move to the fungal pipeline (which is not very different really ...)
Now, for the importing step, can I ask you what is the origin of the fasta sequences you are trying to import? Are these raw sequence files or already denoised file (I am asking because the name you using let me think something already processed but I may be wrong).
All the examples are working with raw fastq sequence-file pairs, which are containing both sequence and quality information, I think it could be possible to import fasta file as well but is not really documented because it is a bit of an unusual case.
A previous port which may be related to your case, if these are already processed sequences:
A previous post to help you importing the sequences file in case they are raw data, is:
Hope it helps,
Luca
In general, what type (and how many) of file do you have for your mock-community sample?