Connect Dropbox app

In this installation guide you can read:

“… php-imagick php-smbclient -y …”

https://english.c-rieger.de/nextcloud-installation-guide-ubuntu-20-04/

But there is no package for Ubuntu 20.04 LTS (Focal Fossa). What a pity.

https://packages.ubuntu.com/de/bionic/php/php-smbclient

I think you need another repository. Not tested. I use Debian GNU/Linux.

cd /etc/apt/sources.list.d
echo "deb [arch=amd64] http://ppa.launchpad.net/ondrej/php/ubuntu $(lsb_release -cs) main" | tee php.list

https://english.c-rieger.de/nextcloud-installation-guide-ubuntu-20-04/

Perhaps read the full manual.

Debian vs. Ubuntu

On Debian Buster GNU/Linux you can use

apt-get install php-smbclient

But is also not in Debian Bullseye (next release).