Update [Oct 1, 4:26 pm MST]: My 3.5.4 update has just been approved. You may still need to make sure that Tap Forms has access to your Contacts though until I get the next update out for that.
Last week I submitted an update to Tap Forms which fixes some bugs that came up with El Capitan. I'm still waiting for Apple to review the update.
The Mac Trial version (which can be licensed for free if you already have the Mac App Store version installed) already has the fixes for El Capitan.
One issue you will see is that in Single Column List View, the records displayed in the list view won't always match the selected record on the right. If you switch to multi-column list view the problem goes away. This is especially evident after a search.
If the main window doesn't load for you on El Capitan:
When you launch a fresh copy of Tap Forms, you are usually asked to allow it to have access to your Contacts database. Tap Forms requires this in order to display the Contacts popover when you have a Contact field in your form. Unfortunately without the required access, the main window won't load. So what you need to do is go to the Security & Privacy settings in the System Preferences app and then make sure Tap Forms has been granted access to Contacts. Then launch Tap Forms and the window will load.
I've submitted version 3.5.5 to Apple for review to fix this issue. The version from the direct download of Tap Forms already has this fix. You can download that version here.