I am running High Sierra. Why can't I install Mojave on my Macbook Pro?

Macbook Pros have been in continuous production since 2006 with very different OS capabilities among the many variants, and you did not provide us the sub-model. You can find your MBP's sub-model by doing "About this Mac..." from your Apple menu:

You must have a Mid 2012 or newer MBP to install Mojave.

If you have the right computer sub-model and either a) cannot find the Mojave installer or, b) encountered trouble with the actual install process, please give us the details and we can see if there are any known issues at play.

• One known issue is that if an older version of the installer is already on the computer (look in the "Applications" folder), the installer may fail to download.
• Another known issue is that FireFox does not appear to be capable of downloading any macOS updaters lately. Use Safari.

To learn how to upgrade macOS to the latest version, please read How to upgrade to macOS Catalina. It also includes instructions for upgrading to macOS Mojave if that is your preference.

I have a early 2015 retina 13 inch, but when I try to upgrade from Apple to Mojave it states it is corrupted and can't be used. What is the next step?

That message usually means there is an older version of the updater already on your computer as I mention above. Remove any older installers before trying again.