"Photos cannot play this video because the resource is unavailable."

Solution 1:

I saw the same issue (what I consider a bug) in Catalina. Mainly iPhone videos recorded with older iPhones. Finding the same video within Photos' bundled library in Finder plays the video without issues and adding them to Photos a second time, again plays those same videos in Photos without errors.

After an update to Big Sur all affected videos play again in the Photos.app

If you really cannot, or do not want to update to Big Sur, you can work around the issue by re-importing the movies into Photos.

You will need the original filename from Photos' Info window, as illustrated below with "IMG_0987.MOV":

Now launch Terminal (which can be found in /Applications/Utilities/Terminal.app, or by searching with Spotlight) and change your directory to the current Photos' library (type cd with an added space at the end and drag the library from Finder onto the Terminal window), and you should end up with something similar to...

cd /Users/`whoami`/Pictures/Photos.photoslibrary

...then press enter. (Note: changing the directory will make it easier to search, especially, when the library resides on an external volume)

Now search for the original filename noted earlier with something like...

find . -xattrname "com.apple.assetsd.originalFilename" -exec xattr -lr {} \; | grep "original_filename.MOV"

(...replacing "original_filename.MOV" {without the quotes} for the original file name)

This may take quite some time and is CPU intensive (fans might become audible), so just be patient. The output should finally come up with the path to the file, including its name, which is a long alpha numeric string (you can stop the search with cmd. once found). Re-import that movie into Photos a second time and it should play within Photos.

About your second question of accidentally deleting QuickTime Player: it's bloody hard to do that in Catalina, since QuickTime Player is a system application on a read-only volume. So I must ask: are you sure you cannot see... /System/Applications/QuickTime Player.app or the firm linked... /Applications/QuickTime Player.app ...in Finder?