After looking at the documentation here for custom buttons using conditional logic, I'm still getting a syntax error for button. I believe it has something to do with attempting to append the URL with the ID. Because the content source is URL, not execute JS, it's not appropriate to use JSENCODE here right?
https://{!IF(NOT(ISNULL(Opportunity.RECAP_ID__c)),
"recap.retailcapital.com/backend/account_report.php?id="&
{!Opportunity.RECAP_ID__c},"recap.retailcapital.com")}
You need to add the Google Info custom button from the Salesforce1 & Lightning Actions bucket to the Salesforce1 & Lightning Actions section in the layout for it to appear in the Lightning Experience view of the page.
If you just move it to the Custom Buttons section (bottom part of the above screenshot), it shows in Salesforce classic view only.
In your screenshot, you added the button to the Detail page, so it shows in Classic only. Add the button to the Salesforce1 and Lightning Experience Actions section just above it.
There are a couple of overlapping challenges here.
JavaScript buttons are not supported in Lightning, so that option is out.
Cases have special behavior in Lightning Experience regarding placement of Quick Actions:
When feed tracking is enabled for cases or work orders, the page-level action menu on those records contains only custom buttons and supported standard buttons. Quick actions appear on the Chatter tab.
If you must have this button shown in the button bar on the Case record page, and you must have feed tracking turned on, I believe only code-based solutions remain available to you.
Those would be either
I would prefer the latter, myself, for a fully-native Lightning experience. However, I don't believe there's a way to do either of these without writing Apex + either Visualforce or Lightning JavaScript.
Best Answer
You can try with following code, there is no need to enclose Opportunity.RECAP_ID__c inside {!}