Alignment using bwa-mem2

Hello

I need help in aligning the sequence with reference using bwa-mem2. I used the following code:

bwa-mem2 mem -t 8 gch38.fa DE98NGSUKBD117612_1_1.fq DE98NGSUKBD117612_1_2.fq > d3_align.sam 

I got the following error:

ERROR! Unable to open the file: gch38.fa.bwt.2bit.64

There is no gch38.fa.bwt.2bit.64 file. I have the following the reference indexes:

gch38.fa  gch38.fa.0123  gch38.fa.amb  gch38.fa.ann  gch38.fa.pac

What is the gch38.fa.bwt.2bit.64 file?

Thank you

Priya