where are the files you use in the project located?
Did you rename any files or move them to a different location (or even rename any folders)?
Did you use any files that were on a removable device that you don’t have connected now?
Some users place the media files they use in their projects from various locations without prior organisation of the files. Since Shotcut does not currently provide file management it is advisable (for more elaborate projects) to take some precautions.
Here I comment on the structure I use to avoid having the files I use in my projects scattered around.
If you save the project and reopen it, you will see a screen with the missing source files in your project.
From there you can indicate where those files are currently located.
If you are able to tell Shotcut the current location of these files, you will be able to continue with the project and export it.
On the other hand, I suggest a change in the request for help so that other people can also help you. A short description of the problem would be better and preferably in English.