php-mbstring : Depends: php7.4-mbstring but it is not going to be installed code example
Example: como instalar ext-mbstring en ubuntu 18.04
deb http://archive.ubuntu.com/ubuntu bionic main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-security main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main multiverse restricted universe