user4471: The sequence is composed of a jpg followed by an mp4. When the jpg is not present, after I hit Command + R, the sequence can render without errors. When the jpg is present, after I hit Command + R, the sequence cannot render and it returns a message "File Error: Unknown file."

user4182: Final Cut Pro may not recognize your file location. Did you change the location of the jpg after you imported it into the sequence? If so, you may have to locate it again. 

user2197: There are three reasons I can think of:

The file is (as Andrew suggested) offline. 
The JPEG doesnt have a .jpeg extention (final cut has problems with files that are missing their extension)
The JPEG resolution is too large. Try sizing the resolution down to under 2000x2000


4471: <p>The sequence is composed of a jpg followed by an mp4. When the jpg is not present, after I hit Command + R, the sequence can render without errors. When the jpg is present, after I hit Command + R, the sequence cannot render and it returns a message "File Error: Unknown file."</p>
 4182: <p>Final Cut Pro may not recognize your file location. Did you change the location of the jpg after you imported it into the sequence? If so, you may have to locate it again. </p>
 2197: <p>There are three reasons I can think of:</p>

<ol>
<li>The file is (as Andrew suggested) offline. </li>
<li>The JPEG doesnt have a .jpeg extention (final cut has problems with files that are missing their extension)</li>
<li>The JPEG resolution is too large. Try sizing the resolution down to under 2000x2000</li>
</ol>
