Hello 
I am trying to build my index to HISAT2 using the following command 

    hisat2-build rn6_ercc/genome.1.ht2,genome.2.ht2,genome.3.ht2,genome.4.ht2,genome.5.ht2,genome.6.ht2,genome.7.ht2,genome.8.ht2 ht2_base

I got the following errors 

>Error: could not open genome.2.ht2

and 

>Error: Encountered internal HISAT2 exception (#1)

Kindly help 
You should build the index with something like:

    hisat2-build genome.fa index_name

Where genome.fa is the fasta file of the genome you feed hisat2-build, and index_name will be the base name of the index files (*.ht2).