installation – Can not install installed APKs

I've created an application in Unity that has been working well for months.

I then decided to reinstall the application because of a problem that later turned out to be a problem with a CSV file that the application had to access.

But after uninstalling the application and wanted to reinstall it, I received the error "Application not installed".

I have tried to install a previous version of the application and this one is installed very well. I then uninstalled it because I wanted to retry the latest version.
But now, I am not only unable to install the most recent version, but also the earlier version.
Whenever I install and uninstall a different version, I can not reinstall that particular version.

I have tried:
– Restarting the phone
– Deleting the application folder in the folder "Android-> Data"
– just about everything I could find looking for help on Google

I'm on a Samsung Note 8 and I recently switched to Android Pie

Edit: I can not create a new version because I will not be home next week.