+1 I have the exact same problem - MacOS Mojave 10.14.6 - PlanOnMac 4.4
ERROR
PlayOnMac Wizard -> Steam
"Downloading Wine: 4.21" -> Followed by
"The download seems to have failed."
Trying to debug, I opened "PlayOnMac wine version manager"
And I got
Available Wine versions:
"Wine packages website is unavailable"