Unable to Create Custom Long Text Area and Rich Text Area Fields for Salesforce Activities

This is expected functionality in Salesforce. Long Text Area and Rich Text Area field types cannot be created for Tasks and Events

An alternative workaround is to create additional Text Area fields on the Activities object. Text Area fields allow you to store a greater amount of text data in Tasks and Events, and are available as a custom field type for Activities.
To create a Text Area field on Activities:

1. Navigate to Setup > Object Manager > Activity.
2. Select Fields & Relationships > New.
3. Choose Text Area as the field type and complete the field wizard.
4. Save and deploy the field.

Here are some Ideas on the Idea Exchange for this functionality:

• Allow custom "Text Area (Long)" fields on Activities
• Allow Rich Text Fields on Activities