A media file will be marked with ""!"" on the timeline when its original file was moved to another folder, deleted, renamed, or transferred to a cloud, or a desktop computer.
If you try to preview or export the project which has the missing file, KineMaster pops up an error message that some files in the project are missing.

In order to solve the problem, you have to find the media file marked with the exclamation mark ""!"" on the timeline.
Once you find the missing file, you will have two options:
1. Put the original file back to its original place if you have the original file somewhere else.
2. Delete the missing file from the project if you don't have the original file anywhere else.

After this, you should be able to preview and export your project normally.