Error while calling bcftools mpileup - Failed to open -: unknown file type
The - is usually used to mean standard input when reading data. So the message would suggest that the second bcftools command fails to read from stdin, so it fails to read the output of the first bcftools command:

$ echo "foo bar" | /ref/software/bcftools-1.8/bcftools call -mv -Oz -o calls.vcf.gz
Note: none of --samples-file, --ploidy or --ploidy-file given, assuming all sites are diploid
Failed to open -: unknown file type


So there is something wrong with either your input file or with the output produced when running the first bcftools command on that input file. We can't really help more unless you share the file.