To je debian stretch?
Skus apt install php7.0-mbstring
skontroluj si /etc/apt/sources.list
deb http://ftp.sk.debian.org/debian/ stretch main non-free
deb-src http://ftp.sk.debian.org/debian/ stretch main non-free
deb http://security.debian.org/ stretch/updates main contrib non-free
deb-src http://security.debian.org/ stretch/updates main contrib non-free
deb http://ftp.sk.debian.org/debian/ stretch-updates main contrib non-free
deb-src http://ftp.sk.debian.org/debian/ stretch-updates main contrib non-free
deb http://ftp.sk.debian.org/debian stretch-backports main non-free
deb-src http://ftp.sk.debian.org/debian stretch-backports main non-free