Hi Fernando,
I strongly agree with Jim's advice - we recommend switching to 18.04 and following the official AtoM installation documentation for 2.6 if at all possible.
If for some reason you have no choice but to use 16.04, I think the biggest issue will be the PHP version. Ubuntu 16.04 includes PHP 7.0 by default, but AtoM 2.6 requires PHP 7.2. This means you will need to manually upgrade the default PHP version to 7.2, and make sure that all the PHP extensions you need to install for AtoM also use the 7.2 version.
Similarly, you'll need to manually install MySQL 8, as Ubuntu comes bundled with options for MySQL 5.7 or MariaDB 10.0. AtoM definitely requires v8.0 for the 2.6.4 release.
There may be other mismatches between the dependencies you need in AtoM and what is included by default in 16.04, so follow the instructions slowly, check the version on anything you install and compare that with what is included by default in 18.04, and then find a way to manually install the correct dependency version whenever there is a mismatch.
As you can see, things will go much easier if you're able to use 18.04 instead - and you'll be using a more secure, maintained O/S.
Good luck!
Cheers,