Applies To: SharePoint Online
By default, whenever you add content to a SharePoint site, it is automatically searchable. However, sometimes, the managed properties are not created automatically, or they are mapped incorrectly. When this happens, it can be extremely frustrating to a site owner.
You will be presented with a less-than-helpful message like this at the top of the page:

Symptom 1: items from a list or library are not showing up in searches
Symptom 2: managed properties for your fields are read-only (disabled) when you try to edit the search schema for a site
Symptom 3: you might have copied the list from another site, using the same field names as the previous site. An excellent example of this is a stage site and a production site.
First, be sure you understand the difference between managed properties and crawled properties.
The managed properties are not getting set up correctly. To resolve the issue, try the following:
Note: You will need to be a SharePoint tenant admin to perform these steps.
Go to the Search Administration in your SharePoint tenant administration.
The URL (at the time of this writing) is formatted as follows
https://yourtenantname-admin.sharepoint.com/_layouts/15/searchadmin/TA_SearchAdministration.aspx
Select Manage Search Schema. This brings up the Managed Properties admin screen. Search for your Managed property.

Select the managed property you are trying to update (but cannot from the site search schema.)

Make sure that the field is searchable by clicking “Searchable”.

Select “Add a Mapping.” This is where you will select the crawled property that your managed property is pointing to:

After you have selected the crawled property, click “OK”.

When you are done with mapping, you may need to re-index the site.
Go to Site Settings. Under Search, select “Search and offline availability.”

Select “Reindex site.”

I hope this post will save you frustration in setting up your SharePoint search.