can't delete kernel

[John Carroll]

Hello all,

I tried to install a software package and was not allowed. It tried to 
remove some old kernels and was unsuccessful and therefore would not 
install the software. I tried the cli and synaptic with same results. 
How can I again install software. Below is output from the cli attempt 
to remove

Removing linux-image-4.15.0-29-generic (4.15.0-29.31) ...
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-4.15.0-29-generic
/etc/kernel/postrm.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 12: /etc/default/grub: iommu=soft: not found
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 127
dpkg: error processing package linux-image-4.15.0-29-generic (--remove):
  installed linux-image-4.15.0-29-generic package post-removal script 
subprocess returned error exit status 1
Removing linux-image-4.15.0-32-generic (4.15.0-32.35) ...
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-4.15.0-32-generic
/etc/kernel/postrm.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 12: /etc/default/grub: iommu=soft: not found
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 127
dpkg: error processing package linux-image-4.15.0-32-generic (--remove):
  installed linux-image-4.15.0-32-generic package post-removal script 
subprocess returned error exit status 1
Errors were encountered while processing:
  linux-image-4.15.0-29-generic
  linux-image-4.15.0-32-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)

thanks, john