Mercury Reader requires an extra php extension outside of what lando's drupal8 recipe typically provides.
Adjust your lando.yml Landofile to include the following:
services: appserver: build_as_root: - apt-get update -y - apt-get install libxslt1-dev -y - docker-php-ext-install xsl - docker-php-ext-enable xsl
Rebuild your lando environment to install the libraries:
lando rebuild -y