Starting with Maple 2018.2 Maple is fully 64 bit and this warning should not occur. If you do receive this warning while installing Maple 2018 it is safe to ignore it and hit OK.
Versions of Maple prior to 2018 can safely ignore this message if you are using Mac OS 10.13 or prior. On Mac OS 10.14 only Maple 2018 and on will be compatible.
For more information, please see Apple's announcement.