Trying to create a .bam file without the need for a .sam file

I’m trying to use the code specified in this link to create a .bam file without the need for a .sam file.

Here is the code I’m using:

bwa mem -R "@RGtID:$SAMPLEtPL:illuminatSM:$SAMPLE" 
    $REF_PATH/hg38.fasta 
    mapping/R1.fastq.gz 
    mapping/R2.fastq.gz | 
    samtools sort - | 
    tee mapping/$SAMPLE.bam | 
    samtools index - mapping/$SAMPLE.bam.bai 
    2>> log

and I get the following error:

[M::bwa_idx_load_from_disk] read 0 ALT contigs
`[E::main_mem] fail to open file ` '.

The ref path and file locations work when creating and converting a .sam to a .bam file.
Any ideas?