The error, system is running in low-graphics mode, happens when booting Ubuntu. One reason for the error is that disk space is exhausted. We can boot Ubuntu in recovery mode and use the df command to check it.
if the disk is full, please use the command to find the largest directory.
du -sh -BM * | sort -g
Please use the command to remove the directory. For example,
rm -rf YOUR-DIR
Sometimes we cannot remove the directory because the file system is read-only. Please remount it by the command. For example,
sudo mount -o remount,rw /dev/YOUR-DISK