Dual boot issues with app store

Hello to everyone,

I would like to instal on my macbook pro 2016 with touchbar a second mac operating sistem. Now I have the last Catalina os and I would like to instal in a apfs volume (or, if it is impossible, on a partition) macos Mojave 10.14.6.

I read a lot in internet but almost everyone told to download from appstore the Mojave installer, but I can't find it in any way possible.

How could I do? I had downlaod it from an non official releaser but during the installation the say that is corrupted.