Já fiz essa instalação umas 10 vezes. De uma hora pra outra parou de funcionar.
esse é o passo a passo que eu sigo:

apt update && apt upgrade
apt install -y composer php-symfony apache2 php7.4 mariadb-server php7.4-{zip,mysql,ldap,curl} curl lsb-release apt-transport-https ca-certificates wget

wget https://getcomposer.org/composer.phar 
php composer.phar create-project "novosga/novosga:^2.0" novosga

no passo do composer,phar dá uns erros que vão alem de minha compreensão:

pi@senhas:~ $ php composer.phar create-project "novosga/novosga:^2.0" novosga
Creating a "novosga/novosga:^2.0" project at "./novosga"
Installing novosga/novosga (v2.0.8)
  - Installing novosga/novosga (v2.0.8): Extracting archive
Created project in /home/pi/novosga
Installing dependencies from lock file (including require-dev)
Verifying lock file contents can be installed on current platform.
Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - novosga/composer-installer-plugin is locked to version v1.0 and an update of this package was not requested.
    - novosga/composer-installer-plugin v1.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.
  Problem 2
    - ocramius/package-versions is locked to version 1.4.0 and an update of this package was not requested.
    - ocramius/package-versions 1.4.0 requires composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.
  Problem 3
    - symfony/flex is locked to version v1.2.0 and an update of this package was not requested.
    - symfony/flex v1.2.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.
  Problem 4
    - ocramius/package-versions 1.4.0 requires composer-plugin-api ^1.0.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.
    - ocramius/proxy-manager 2.1.1 requires ocramius/package-versions ^1.1.1 -> satisfiable by ocramius/package-versions[1.4.0].
    - ocramius/proxy-manager is locked to version 2.1.1 and an update of this package was not requested.

Use the option --with-all-dependencies to allow upgrades, downgrades and removals for packages currently locked to specific versions.

ocramius/package-versions only provides support for Composer 2 in 1.8+, which requires PHP 7.4.
If you can not upgrade PHP you can require composer/package-versions-deprecated to resolve this with PHP 7.0+.