QuickTime - 'select all' greyed out

Hello Michael Vallance,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with copying a short movie to the end of another movie. As the formatting isn't the same for both clips, this may explain why Quick Time won't allow you to select that option. You can find some information about adding a movie clip here:Add a movie or clip in QuickTime Player on Mac - Apple Support

"Add a new clip

1. In the QuickTime Player app  on your Mac, open the movie that contains the clip you want to add. 
2. Choose View > Show Clips.
3. Select the clip, then choose Edit > Copy.
4. The selected clip is outlined in yellow.
5. Open the movie that you want to add the clip to, then choose View > Show Clips.
6. Choose Edit > Paste.
7. The clip appears at the beginning of the movie. Drag the clip to rearrange it.
8. When you’re finished, click Done."

We hope this helps.

Cheers!