: If you are using a 64-bit version of VirtualDub, you must have the 64-bit version of the codec installed. A 32-bit codec will not be visible to a 64-bit instance of the software.
If using specific codecs like PICVideo MJPEG, try unchecking as this can sometimes block initialization. Use VirtualDub2 : Unable To Initialize The Output Video Codec Virtualdub
If the source file is corrupted or in an unsupported format (like MKV or MP4), use tools like Handbrake or VLC Media Player to convert the file to a standard AVI format before importing it into VirtualDub. : : If you are using a 64-bit version
: Installing multiple codec packs can lead to registry conflicts that prevent VirtualDub from correctly identifying drivers. Troubleshooting Steps Install VFW-Compatible Codecs : Unable To Initialize The Output Video Codec Virtualdub