Greetings,
I recently had the same issue which I just resolved. For this error you must go and change each individual foliage mesh settings. Change the setting to following.
Min Lightmap Resolution > 8
Source lightmap index > 1
Light map resolution > 8
These settings resolved my problem after two days of research. Yet again I’m just a beginner in ue4. My answer may not be 100% accurate for every type of foliage. 