This underlying tool for ASTC encoding was recently updated, and the command line arguments were changed on some of the options. This causes PVRTexToolCLI to fail when calling it to encode. Since I can’t see the command line that PVRTexTool sends, this is likely easier to track down and fix on your end.
This was on macOS Catalina 10.15.6, and when installing the new astcenc you have to chmod 755 on it, and launch it once from Finder to create a security exception for it. Otherwise Catalina will block running astcenc. I renamed astcenc-avx2 on an avx2 capable MBP 16".