In Videopad, with the project on the timeline, when you Export as a Video File the usual default is mp4, though many other formats are available.
As illustrated in the earlier image, click the Export menu tab, then the Video File button on the toolbar.
Look over the settings, which are usually fine as is, then Create. The resulting mp4 will normally go to the Downloads folder on your PC.
That's the file needed for the YT uploader.