Unable to track Field History of Formula fields

To track the value of a Formula field via Field History, create a custom field of the same data type, populate it using a Record-Triggered Flow, and then enable Field History Tracking on that custom field.

Please follow the below steps:

1. Create a Custom field with the Formula field's Data type. More info on how to Create Custom Fields.

2. Create a Flow to populate the Custom field with the value of the Formula field:

In Lightning Experience:

a) Click the Gear icon | Click Setup | Flow | New Flow | Record Triggered Flow | Create
b) Select the object from the Object look up field
c) Evaluation Criteria: Created, and every time it's edited.
d) Choose Actions and Related Records as you are updating a field on the record that triggered the flow
e) Click Done
f)  On the flow builder click the "+" between the start and end nodes and select Update triggered Record

• Fill out the label - api will auto populate when you click out of label
• Use the case record that triggered the flow
• Condition Requirements to Update Record - "None- Always update record"
• then search the new custom field to populate in the Field look up, and choose the formula field in the "Value" look up
• Click done
• Click Save 
• Click Activate

      
3. From the Management Settings for the object, click Set History Tracking and check the Custom Field to track.

In Lightning Experience

From the Fields & Relationships Settings for the object, click Set History Tracking and check the Custom Field to track. 

NOTE:

Field History Tracking supports 20 fields per object out of the box. With Field Audit Trail (FAT) add-on, customers can also increase the number of tracked fields to 60. If the customer already has FAT and/or Shield and needs to track even more than 60 fields, there is a possibility up to 100 fields per object using this SKU is available upon request and approval from PM.
 

Note: 

Please note the support of the formula is limited as described in this article.