Custom field is not available as an option in a Saved Search Results tab > Columns subtab or any of the tabs (Results, Available Filters). The custom field is applied to the correct record type (Applies To tab) and the Display Type is set to Normal. Store Value checkbox is checked in the custom field.
To resolve this, set the Default Level for Search/Reporting of the custom field to Edit. To do this, perform the following steps:
1. Navigate to Customization > Lists, Records, & Fields > Type of Custom Field (e.g. Entity Fields).
2. Edit the custom field.
3. Click the Access tab.
4. Set Default Level For Search/Reporting field to Edit.
5. Click Save.