Hi Almaz,
If Area_Acres is a numeric type field, you will need to change it to:
str([Area_Acres]) + '/n' + 'Acres'
Otherwise its seems there is a problem with your install of the numpy module. ArcMap 10.3 is supposed to use the 1.7.1 version and is supposed to be installed on your computer with 10.3 if it is not detected, though if you did a custom install, you could have skipped it.
ArcGIS 10.3.x for Desktop system requirements—Help | ArcGIS for Desktop
If this is indeed the issue, you might look at downloading and installing the module seperately instead of reinstalling ArcGIS 10.3
numpy 1.7.1 : Python Package Index