I keep getting this message and I don't know how to resolve it.

That is the only time I see this message. It may happen more for me since I have audio hardware clocked outside of my Mac. If my audio hardware is clocked to 44.1kHz and the project I load is 48kHz, this is the message I see. I have to manually change my clock, or change the setting on the project. I see this message about a dozen times a day :(

I can see if your interface follows the Logic project setting that you would not see this message.

Another thing the OP can check is to see if his hardware interface is set to External clocking with no external clock present (i.e., is it clock sync'd).

What interface are you using?

Check to be sure your audio interface clock is set to internal...

In the UA Console app, what is the status of the sample rate and clock settings (lower left corner of the window)?

You will see this message if your project's audio sample rate is not the same as the sample rate your hardware is using.

Go into Logic -> File -> Project Settings -> Audio... and check the Sample Rate there.

It should be set to match your hardware.

Ah, OK, you have actual external hardware - OK. However, this message also pops up (very regularly) with users who have no external hardware. I think your case is the first ever in which the message actually was true in the sense that there is actual external hardware involved. My bad.

And what is the project sample rate in File -> Project Settings -> Audio?

Also, is the Apollo set as your audio interface in Logic Preferences?

If the project is 44.1kHz along with the device, it should be syncing up...

You will see this message if your project's audio sample rate is not the same as the sample rate your hardware is using.

Go into Logic -> File -> Project Settings -> Audio... and check the Sample Rate there.

It should be set to match your hardware.

Nope. That is rarely the case when you get this message (never, really - in the over ten years I've been on this site this never once this was the case, and this has been asked thousands of times - just search this site for "error while trying to synchronize" or "error synchronize")

What happens is that Logic (or better: a software instrument (Alchemy may do this, for instance)) cannot "keep up" with the playback speed, and it drops samples - the number in the error message is ALWAYS lower than whatever the set samplerate of the project is. It is in fact a variation on the "system overload" message. Your synth patch is too much for your CPU.

There are some remedies:

1. It often works to deselect whatever instrument track you have selected by selecting (any) audio track. Also make sure the instrument track is NOT record armed (R button).
2. It may also help to set the synth or instrument on the selected track to a lower resolution - if it has such a setting.
3. Another option is to bounce the track in place (BIP) and power off the instrument.
4. What also works is to freeze the track