At first create a Ghost Content key and then open Ghost admin dashboard again.
Go to Settings > Code Injection
Paste the following code with your updated content key
<script>
var themeConfig = {
ghostSearchKey: 'ADD_YOUR_CONTENT_KEY_HERE',
includeBodyInSearch: true
};
</script>
Make sure you used your own Content key there for ghostSearchKey option.
Example :
<script>
var themeConfig = {
ghostSearchKey: '5cc707e097ada98f2a869daf25',
includeBodyInSearch: true
};
</script>