php-gd is installed and listed in phpinfo() but still getting Uncaught Error: Call to undefined function imageftbbox() which means the only other requirement, freetype, was not compiled in?
Generally you would pass something like –with-freetype-dir=/usr/include/freetype2 at compile time. From phpinfo() it seems that either wasn't passed or it's not logged, but either way, these are the reported compile time flags:
'./configure' '--enable-static=' '--enable-fpm' '--enable-gd' '--enable-soap' '--enable-opcache' '--enable-mbstring' '--enable-pdo' '--enable-ftp' '--enable-sockets' '--enable-exif' '--enable-bcmath' '--enable-calendar' '--enable-shmop' '--enable-pcntl' '--with-pdo-mysql' '--with-openssl' '--with-curl' '--with-zip' '--with-zlib' '--with-png' '--with-gettext' '--with-xmlrpc' '--with-mysqli' '--with-pcre-regex' '--with-jpeg' '--with-freetype-dir' '--with-bz2' '--with-xsl' '--with-iconv' '--without-pear' '--without-sqlite3' '--without-pdo-sqlite' '--prefix=/home/vsts/work/_temp/local-lightning-php/php-7.4.1/build'