composer install
...
squizlabs/php_codesniffer 3.2.3 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.
and
composer require simplexml
...
Could not find package simplexml.
Did you mean one of these?
dkrnl/simplexmlreader
imsop/simplexml_debug
adduc/cdata-simplexml
dmx/simple-xml-parser
dataground/simpledom
simplexml is a PHP extension, not a Composer package, so you can’t install it using Composer. On Debian, you apparently need to install the php-xml package.
and