Why this error occurring all time
first, check app.txt under the sites folder
also check repository exists or not
First You need to check if app is installed or Not.
bench --site your-site list-apps
If you see print designer then run
bench setup requirements
This Basically will Install all the packages if not installed
(Optional) if Issue Remains Persistant
Run Bench Migrate
bench --site your-site migrate
You need to install demo_app i think