When trying to click on the box to enable the endpoint correlation, the checkmark briefly appears, but is very faded. Then a popup screen appears displaying "Internal error." in the ATP UI. When adding the SEPM DB and clicking on "Save" in the lower left portion of the settings screen you see in red 'one already exists' even though the list is empty. Then the faded checkmark disappears.
Open a Support ticket with Symantec Support. Provide the logs from the ATP appliance or Virtual Machine (VM) by running the 'gather_logs' from the command line (CLI) as the 'admin' account. Provide the Support team with the Universal Unique Identifier (UUID). This can be obtain from the (CLI) by running the 'bsupport'. Take a screen shot or cut and paste the (UUID) in the Support case that you have open. Also proivde any screen shots of the actual error that you are seeing.