Error 'This custom field is referenced elsewhere in salesforce.com: AI Application' in Opportunity custom fields

To resolve this issue, identify which AI Applications are being referencing the custom field, and then temporarily disable the feature to delete the references. In Salesforce the AI applications are the following Einstein features:

1 - Identify the Installed AIApplication Type(s) by finding the Einstein feature referencing the custom field

Einstein Feature

Einstein Forecasting

Einstein Lead Scoring

Einstein Opportuntiy Scoring

Einstein Prediction Builder

Einstein Discovery

Recommendation Builder or Next Best Action

2 - Modify, Disable or Delete the Einstein Feature(s)

For example if you identified the custom field is in use in 'Einstein Prediction Builder', then to delete the records with the reference to the custom field with these steps:

To delete 'Einstein Prediction Builder'

1. From Setup, enter Einstein Prediction Builder in the Quick Find box, then select Einstein Prediction Builder. Or, on the Einstein Prediction Builder tile in Setup Home, click Get Started.
2. If it’s your first time using Einstein Prediction Builder, click Get Started on the splash page.
3. Delete existing Einstein Predictions.
4. After deleting all existing Einstein Predictions, try changing again the Opportunity Custom Fields field type

Note: Since this includes deletion of customization, practice caution when deleting anything in Salesforce.

See also
Einstein Prediction Builder
Considerations for Setting Up Einstein Prediction Builder