Following the instructions on https://docs.pressbooks.org/installation/, I found that I also needed to install the php extension exif (in addition to xsl) in order to get exports to work (I’m guessing because of images generated by QuickLatex).
I also found that:
define( ‘PB_EPUBCHECK_COMMAND’, ‘/usr/bin/epubcheck’ );
should be the ‘jar’ version shown later in the same documentation:
define( ‘PB_EPUBCHECK_COMMAND’, ‘/usr/bin/java -jar /somepath/epubcheck.jar’ );
perhaps the documentation should be cleaned up to be more self-consistent.