ALOS PALSAR L1.5 Mosaic

Hello! I was hoping someone could help me out with an issue I am having when I try to apply the Mosaic tool to ALOS-PALSAR L1.5 data, which is already georeferenced.

I have radiometrically calibrated my images and now want to mosaic them. My area of interest is very flat and I’ve learned from literature that the SRTM DEM is not very accurate. With this information, I thought it would be best not to apply a terrain correction to my image. However, when I try to mosaic my images, I receive an error message that it must be terrain or ellipsoid corrected first. When I try to apply these correction algorithms, I get an error message saying that my source image should not be map projected.

Does anyone have any advice on how I can move forward? Thank you!