I figured it out for my project.
Open your project settings in the search at the top put in the word alpha this should filter the options so you can see the top choice:
post processing
The first choice should be Enable alpha channel support in post processing (experimental)
When you see this drop down box it was disabled, for me I choose the first choice not knowing what it did, (LINEAR COLOR SPACE ONLY) and my project built. This is all I changed to fix it.
It requires a restart of the editor and will recompile all of your shaders.
Please note the 3rd choice is (ALLOW THROUGH TONEMAPPER) I haven’t tried this choice it might also work.
Added note: What this does is change the word FALSE to the number 1 in the defaultengine.ini under the setting r.PostProcessing.PropagateAlpha= Was the word (FALSE) requires the numbers 0,1,2 for the different settings mentioned above in the drop down menu. I am 95% sure this changed because they finished this feature, and now had the new options that were not required in previous versions of the engine.