When trying to save an Experiment Installer within LabAuthor, I get a message indicating some resources are missing and the installer fails to save. How do I resolve this?

When saving an Experiment Installer, for uploading an Experiment to your LabTutor Server, LabAuthor requires that any Missing Resources be replaced:
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The most common cause of missing resources are Images / Videos / Data Files that are part of an Imported Content / Background / Popup Page from another Experiment. 

When importing Pages from other Experiments, LabAuthor does not automatically import associated Page Items (such as Images) that are on that Imported Page. These need to be manually added, using the Resource Menu on the left-hand side of LabAuthor. Missing items are indicated with a red italic typeface:

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To replace the missing item, right-click and select 'Replace Missing...', and navigate to the location of the missing item (which will be within the same experiment folder as the original Imported Page):

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It is recommended that whenever a Page is imported from another Experiment, that you immediately check and replace any missing resources - it is much more difficult to do this further down the line, as you would need to re-locate the Page and Experiment the Imported Page was taken from.