This is caused because the user is running as qemu for virt-resize and if qemu does not have privileges to read from the source and write to the destination, it will fail with the below. So either change the uid of qemu or change the ownership of the source and target.
Solution:
export LIBGUESTFS_BACKEND=direct
virt-resize --expand /dev/sda2 /root/kvmtemplates/windows2019-eval-template.img /root/kvmguests/kvmkvmuser451511/kvmkvmuser451511.img
[ 0.0] Examining /root/kvmtemplates/windows2019-eval-template.img
virt-resize: error: libguestfs error: could not create appliance through
libvirt.
Try running qemu directly without libvirt using this environment variable:
export LIBGUESTFS_BACKEND=direct
Original error from libvirt: Cannot access backing file
'/root/kvmtemplates/windows2019-eval-template.img' of storage file
'/tmp/libguestfsFNamzn/overlay1.qcow2' (as uid:107, gid:107): Permission
denied [code=38 int1=13]
If reporting bugs, run virt-resize with debugging enabled and include the
complete output:
virt-resize -v -x [...]
virt, resize, libguestfs, appliance, libvirt, user, qemu, privileges, destination, uid, ownership, expand, dev, sda, kvmtemplates, eval, template, img, kvmguests, kvmkvmuser, examining, variable, export, libguestfs_backend, backing, tmp, libguestfsfnamzn, overlay, qcow, gid, int, reporting, debugging, enabled, output,