I was unable to download the data using the following command:
humann_databases --download utility_mapping full .
The output was:
Creating subdirectory to install database: /mnt/utility_mapping
Download URL: http://huttenhower.sph.harvard.edu/humann_data/full_mapping_v201901b.tar.gz
CRITICAL ERROR: Unable to download and extract from URL: http://huttenhower.sph.harvard.edu/humann_data/full_mapping_v201901b.tar.gz
It seems that the server might be down, as I am also unable to access the website: https://huttenhower.sph.harvard.edu/