If you run "apt-get install lsphp73-pear", did it show it has been installed successfully or failed?
What this command returns: "dkpg -l | grep lsphp73", does it include lsphp73-pear.
		
		
	 
 Install was successful.
ii  lsphp73                          7.3.22-1+stretch                                amd64        server-side, HTML-embedded scripting language (LSAPI binary)
ii  lsphp73-common                   7.3.22-1+stretch                                all          Common files for packages built from the PHP source
ii  lsphp73-curl                     7.3.22-1+stretch                                amd64        CURL module for PHP
ii  lsphp73-dev                      7.3.22-1+stretch                                amd64        Files for PHP70 module development
ii  lsphp73-igbinary                 3.0.1-1+stretch                                 amd64        igbinary PHP serializer
ii  lsphp73-imagick                  3.4.4-1+stretch                                 amd64        Provides a wrapper to the ImageMagick library
ii  lsphp73-imap                     7.3.22-1+stretch                                amd64        IMAP module for PHP
ii  lsphp73-intl                     7.3.22-1+stretch                                amd64        Internationalisation module for PHP
ii  lsphp73-json                     7.3.22-1+stretch                                amd64        JSON module for PHP
ii  lsphp73-memcached                3.1.5-1+stretch                                 amd64        memcached extension module for PHP, uses libmemcached
ii  lsphp73-msgpack                  2.0.3-1+stretch                                 amd64        PHP extension for interfacing with MessagePack
ii  lsphp73-mysql                    7.3.22-1+stretch                                amd64        MySQL module for PHP
ii  lsphp73-opcache                  7.3.22-1+stretch                                amd64        Zend OpCache module for PHP
ii  lsphp73-pear                     1.10.10-1+stretch                               all          Provides PHP PEAR package
I tried switching to 7.4 and got the same result.