On Windows 10, Microsoft Windows Defender has a Controlled Folder Access feature than can sometimes prevent applications, including SliQ Invoicing, from writing to folders on your PC.
If you find that SliQ is unable to install, e.g. complaining that it can't write a temporary file, or that after installing successfully SliQ is unable to save registration details when you have entered your unlock code, try temporarily turning of Windows Defender Controlled Folder Access and see if you can then complete installation/ registration.
To turn controller folder access off see this link on Microsoft’s site:
Only turn off Controlled Folder Access temporarily. If turning controller folder access off lets SliQ work and register OK then add SliQ to Microsoft Windows Defender’s whitelist so that it can continue to work even with Controlled Folder Access enabled. See this help link:
You could add an exclusion for the Documents\SliQInvoicing5 folder for example.