You're working on an XFS filesystem, in this case you need to use xfs_growfs instead of resize2fs. Two commands are needed to perform this task :
# growpart /dev/sda 1
growpart is used to expand the sda1 partition to the whole sda disk.
# xfs_growfs -d /dev/sda1
xfs_growfs is used to resize and apply the changes.
# df -h