I just had this issue myself. I made sure there was no alpha channel in the ‘output layer’ of my last node. I then connected the ‘Write’ node to the last node of the series. I outputted as a series on PNG’s that will open in Gimp.
I also rendered to MP4 part 14 using the hev1 /HEVC codec.
You may need to try some different codecs but the scroll-over help boxes tell you if the codec will work or not.