Hi Ellen,
This appears to be a problem with the PHP version packaged in 16.04. One of our developers has reproduced the error, but noted that apcu is still successfully installed, and this shouldn't prevent you from continuing with the installation. If you ignore the error and configure with the rest of the steps in that section, then both apc and apcu should be installed.
If you try running the following command after completing the steps (ignoring the error), you should see apc and apcu listed as installed:
Right now apcu-bc is not officially supported in Ubuntu, but AtoM doesn't work with the default version of apcu supported as a package. The PECL version we ask you to install instead does throw an error, but if you ignore it, you should be able to proceed.
The long-term solution will be for us to upgrade AtoM's cache layer so it can work with current versions of APCu, which Ubuntu maintains as package. I have filed a bug for this here:
Let us know if you are able to finish the steps to configure apcu-bc and proceed with the installation.
Cheers,