Trying to create a .bam file without the need for a .sam file
This is an error from bwa saying that it can't find your input files:
Here's a checklist:

Check that you have no whitespace after your \ newline breaks. I.e. \ is the very last character
Check that you have no whitespace in any path names
Quote your variables: $REF_PATH -&gt; &quot;$REF_PATH&quot;
Use set -x at the top of your bash script to debug.
In general, read this