You're applying the command
e2fsck, which operates on an ext2/ext3/ext4 filesystem, to a FAT32 filesystem. Unsurprisingly,
e2fsck complains that it doesn't find a valid ext filesystem.
If you want to check that filesystem, run
fsck (which will determine the proper utility for the filesystem on that device) or invoke the proper utility directly (
If you still get a mount error, copy-paste the error message. Note that Linux's filesystem driver is sometimes more picky about damaged media than Windows's. If you think the stick might be damaged, copy the data off it with ddrescue :
sudo ddrescue /dev/sda1 /var/tmp/sda1
You can then mount the files in the image with
sudo mount -o loop,ro,umask=022 /var/tmp/sda1 /mnt
By the way, is
/dev/sda really your stick? Usually
/dev/sda is the first internal disk.