Before you continue, please make sure that Soundpad and Steam aren't run in compatibility mode. Start with step 1 and retry the action, that led to the error. If it doesn't help, then try the next step.

1. Run Soundpad as administrator. Here is an instruction.
2. For Steam edition only: Download and install the standalone version. The installer ensures, that all permissions are set correctly and will do the necessary changes to your system. Although the Steam installer should do these things, it apparently doesn't always happen and users reported, that installing the standalone version fixed the problem.
3. Turn off anti-virus.
4. Check for missing or corrupted system files with Microsoft's system file checker tool.

Still not working

If it still doesn't work, then you are out of luck, and we must assume, that Soundpad is installed on a corrupted installation of Windows. Soundpad does casual system calls to register its driver extension. If these calls are blocked, even though you ran Soundpad as administrator, turned off anti-virus software or installed the standalone version, then your Windows version is not behaving as it is supposed to.