I have checked and rechecked this line and I am imputing it exactly. What am I missing? Do I need to install an older version of Armory first?
There is no need to do so. The root keys generated by the old versions of Armory are supported by the newer versions and they should work.
How did you make your backup? Did you print that or did you write it down manually? If it's the latter, maybe, you made a mistake when writing the root key.