the error in mounting is:
couldn't mount RDWR because of unsupported optional features (4000)
according to this source; I found out that Android 10's system images are formatted with EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS. It will deduplicate blocks in the filesystem to reduce the image size. On my Pixel XL, the partition is physically not large enough to successfully run 'e2fsck -E unshare_blocks'.
so to resolve this problem you can use:
e2fsck -E unshare_blocks
but before that you should resize and increase free size of image (because its need beeger than prevois) so you can use:
resize2fs ./system_a.img 1200M
and then
sudo losetup -P /dev/loop3 ./system_a.img sudo mount /dev/loop3 ./system -rw
