problems installing mathtools

Had the same issue. For me it worked after I synchronized manually with the repository using the MikTeX Package Manager (Admin) tool and under the Repository menu using the synchronize.

Then the issue is resolved and the mathtools package appears under the package list.

In cmd type mpm --verify=mathtools that will tell if mathtools is installed, not installed or unknown.

If mathtools is not installed: in cmd type mpm --install=mathtools to install mathtools.

If mathtools is unknown: in cmd type mpm --find-updates or mpm --update-db and then install.

Alternative: in cmd type mpm to start MiKTeX Package Manager then Repository->Synchronize

Then search for mathtools -> select row mathtools -> right click -> install.

The mh package has recently been split into mathtools and breqn, with different maintainers, and they certainly appear in MiKTeX Package Manager:

Which version of mpm do you have?