You are seeing a Library preview. Is the original photo in a folder on an external drive? If so is the drive connected to your computer. LR will need the original in order to create a smart preview for sync with LR mobile.
Open the Metadata panel on the right whilst the photo is selected (highlighted) and click the right-pointing arrow icon next to Folder. That will take you to the correct folder in the left hand panel. You can right-click on the folder name and choose "Show in Explorer" to check if the photos is available. The folder will have a question mark if it is off-line or if it has been moved.
I seem to be having a similar problem but I can preview my photo in LR Develop but an error comes up saying that it cannot sync to Lightroom Mobile as "Photo is missing and has no smart view. It cannot sync until the photo is located" I have gone into the Meta Data and it names the folder the original photo is held so I don't know what to do next. Would appreciate your help.
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Hello, I just got it working! I'm not sure what the problem was but I closed Lightroom and left it for a while and just reopened it and it synced
Thank you for responding