Clone button unavailable when you view Activities in Salesforce Classic

Salesforce Classic does not support the Clone button. Instead, use the Save & New button which serves the same function and purpose as the Clone button. 

When you click these buttons, you'll be directed to new activity record that has the following standard field's values already populated from the previous activity record's details:

• Assigned To
• Subject
• Related To
• Name

Notes:

• These buttons are only available in Salesforce Classic. Full feature parity for these buttons is not yet available in Lightning Experience. See the "Follow-Up Events" section of the Productivity Features: What’s Different or Not Available in Lightning Experience documentation for additional details.
• Clone button for Events is not currently available in Lightning Experience. Please upvote this idea for this functionality. https://ideas.salesforce.com/s/idea/a0B8W00000GdeM7UAJ/add-clone-button-on-evemt
• The button "Save & New" is also available in Lightning Experience however it does not function the same as in Salesforce Classic. Only the value of the Subject field is carried over to the new task. An alternative would be to use the "Create Follow-up Task" Lightning action instead as this action copies the following information from the original task and works similarly as the "Save & New" button in Classic:
  • Assigned To
  • Subject
  • Related To
  • Name

See Also:
Missing 'Create Follow-Up Task' button not available on Events
'Create follow-up event' + 'Create follow-up task' buttons missing in Lightning
Create Follow-Up Tasks