[SalesForce] Add to Campaign Button missing on campaign influence related list

The Salesforce influence model attributes 100% revenue credit to the primary campaign and 0% to any other campaigns users assign to an opportunity. Only the Salesforce influence model supports auto-association. Auto-association adds campaigns to opportunities automatically when the contact on an opportunity is also a member of a campaign.

To attribute revenue more flexibly to campaigns, you or your partners can create custom influence attribution models. For example, create a model that evenly distributes credit for an opportunity across all campaigns that touched it. Or create a model that gives 100% credit to the first or last campaign that touched an opportunity. With Customizable Campaign Influence, you can create a model for virtually any scenario that suits your business. If you’d like to prevent your sales or marketing users from adding or editing campaign influence records, you can lock any of your attribution models. Records for locked models can only be updated via the API."

https://success.salesforce.com/answers?id=9063A000000pBFVQA2#ext-gen25