Hi Rick,
Thank you for the crisp response! I'm still pretty new to AE so this sort of feedback helps a lot for the future.
Will be sure to show layer and mode toggles in the future.
Your trouble shooting methods also works like a charm! Please see my comments in red below.
EDIT/Solution/TDRL;
I created another background layer with the same specs and turned the old one off. Using the newly created one works! The reason? No clue but it works so I'll take it! Your trouble shooting technique was a massive help, thank you so much! I'll be sure to use your technique for future projects

Here's how I would troubleshoot the problem. Step 1, select the first two icons at the bottom left corner of the timeline so you can see both the switches and the modes columns. Figure out where the preview fails. If you can turn off the Glow layer or disable the FX switch in the Switches column for layer 3 and your preview works move on to the next step.
Yep can confirm that turning off (toggling the "eye" looking icon) the glow layer does make the preview work. Toggling the eye icon back on for the glow layer does give the error prompt (same message in my original post) and preview shows only a blank black preview.
Turn glow back on and if or when it fails when all the layers are under the CTI then start right under the Glow layer and start turning off effects. If it fails when layer 2, Saber is active then turn off the FX in layer 2. Keep going through the stack of layers, turning off all effects one layer at a time until you find which layer consistently causes the failure. When you have found the layer, press the u key twice and check all effects by turning them off one effect at a time. If the layer that causes the problem is a nested comp you'll have to open the nested comp and start turning off effects or resetting values.
Awesome, I think we found the culprit! After going one by one, turn layers off I found that the background layer was consistent at giving the error. I even tried turning all layers on again and turning specifically the background layer off and the preview works. Turning the background layer back on will give that same error. After turning background off, I then turned the Fx button off (this is called Gradient Ramp), turned background back on and the preview works. It seems that whenever I toggle the Fx button for Gradient Ramp is what causes this problem. My folowup question: Is there any advice on what I could do to fix the background? I'm sure once I get that correct then the error prompt would be gone and I can finally move forward with this project.
Edit: I created another background layer with the same specs and turned the old one off. Using the newly created one works! The reason? No clue but it works so I'll take it!