This is a codec issue. Convert your source video to WMV (Windows Media Video) using a free converter like HandBrake before importing. WMV is the native format for WMM and runs perfectly.
Since official downloads are no longer available, users must rely on web archives. However, extreme caution is required, as many unofficial sites bundle the software with malware or demand "registration fees" for what should be a free tool. windows movie maker web archive
On the item’s page, find the Download Options section (right sidebar) and click: This is a codec issue