You can opt to use the old activation UI instead of the new activation UI. 

To configure old activation UI

  1. Go to Setup > Object Manager > Agreement > Fields & Relationships.

  2. Click New and select Formula.
  3. Click Next.
  4. Enter Activate_Old in the Field Label field and select Text.
  5. Click Next.

  6. Enter a formula.

    Sample

    IF (OR(ISPICKVAL(Apttus__Status_Category__c,"In Signatures"), ISPICKVAL(Apttus__Status_Category__c,"In Filing"), AND(ISPICKVAL(Apttus__Status_Category__c,"In Authoring"),ISPICKVAL(Apttus__Source__c,"Other Party Paper"))), IF(OR($User.UIThemeDisplayed == 'Theme4d', $User.UIThemeDisplayed == 'Theme4t'), HYPERLINK("/apex/Apttus__LightningAgreementActivate?id=" &Id, IMAGE("/resource/Apttus__Button_Activate", "Activate"),"_self") , HYPERLINK("/apex/Apttus__AgreementActivate?id=" &Id, IMAGE("/resource/Apttus__Button_Activate", "Activate"),"_self")),NULL)
    CODE

  7. Configure the APTS_CustomLinksforActionPanel admin entry to view the new activate button in the Additional actions section of an agreement. For more information, see Admin Entries.