Nextcloud 15.04.14 is running on Debian 9, php7.0 and Apache2. On other Vm there is the Db (MariaDB).
I tried to update php7.0 to php 7.4, without success. I tried to update Debian 9 to Debian 10 without success too.
Do you think it is possible to install Nextcloud 21 and then import data and db even if it is for 15.04.14 ?
Or, is there a better way ?
The problem is the fact that you must upgrade from 15, 16, 17, 18, 19, 20 to 21.
If you only use very little database data (e.g. public shares, users, …) then you can better download all data from the data dir, reinstall Debian 10 or Debian 11 (it it almost stable) with a new nextcloud, MariaDB, … include the data and make all database data new (e.g. public shares, users, …)
Make first a backup of all data.
Ok, thank you. I will try to make php7.4 work. And if it will not possible, I will start from zero, keeping only the datas.
Please do not install e.g. php 7.4 from backports.
Please dist-upgrade your Debian to Debian Buster or Debian Bullseye.
php7.3 Debian Buster: Debian -- Informationen über Paket php in buster
php 7.4 Debian Bullseye: https://packages.debian.org/de/bullseye/php
dist-upgrade your Debian
Ok, I will try. Thank you
You can use PHP 7.3 (Buster) or PHP 7.4 (Bullseye) for Nextcloud 21.
Perhaps you can use PHP 8 from other sources or sid 
PHP (7.3 or 7.4)