It's a test computer.
If you turn off Smart App control, it is not possible to turn it back on, just by reinstalling Windows again.
A fresh install of Windows was done, it didn't have any version of Paint.NET on it before.
Looking at the eventviewer, have these 2 messages at the time I tried to install.
Application: paintdotnet.exe
CoreCLR Version: 6.0.822.36306
.NET Version: 6.0.8
Description: The process was terminated due to an unhandled exception.
Exception Info: System.IO.FileLoadException: Could not load file or assembly 'C:\Program Files\paint.net\paintdotnet.dll'. Your Organization used Device Guard to block this app.Contact your support person for more info. (0x800711C7)
File name: 'C:\Program Files\paint.net\paintdotnet.dll'
Product: paint.net -- Error 1722. There is a problem with this Windows Installer package. A program run as part of the setup did not finish as expected. Contact your support personnel or package vendor. Action _...si_x64_scd_aot_paintdotnet_exe, location: C:\Program Files\paint.net\paintdotnet.exe, command: /setupActions /install DESKTOPSHORTCUT=1 PDNUPDATING= SKIPCLEANUP= "PROGRAMSGROUP=" /disablePGO /skipEstablishNVProfile /skipRepairAttempt