Zap the NaN's: str(arcpy.Describe(r"%buffer fc%").Extent).replace("NaN","")
Do not forget to set the Calculate Value output data type to Extent so you can connect it to the tool's parameter of type extent.
This validates OK for me. No promises whether it actually gets the tool to give you the output you want though!
If it needs to be WGS84, run your buffer with an output coordinate system setting of WGS84 and use that to calculate your extent.
Executing (Calculate Value): CalculateValue str(arcpy.Describe(r"efillcells_poly").Extent).replace("NaN","") # Extent
Start Time: Wednesday, December 1, 2021 5:29:49 PM
Value = 622280.000000002 4857190 626460 4862460
Succeeded at Wednesday, December 1, 2021 5:29:50 PM (Elapsed Time: 0.03 seconds)
Executing (Layer To KML): LayerToKML efillcells_poly C:\gisclass\efillcells_poly_LayerToKML.kmz 0 NO_COMPOSITE "622280.000000002 4857190 626460 4862460" 1024 96 CLAMPED_TO_GROUND
Start Time: Wednesday, December 1, 2021 5:29:50 PM
Succeeded at Wednesday, December 1, 2021 5:29:51 PM (Elapsed Time: 1.91 seconds)