It looks like you‘re trying to trying to overwrite one of your project files (sansteleportsandkillsyou.pclx) with your exported video. Please instead save your exported video with a separate file name and make sure to use the standard file name extension for the file format you selected (e.g. .mp4, .avi or .webm) and not one of Pencil2D’s file name extensions (.pcl or .pclx).