Apparently, there seem to be many people who are getting the following error when you try to build Lazarus:
Unable to find file "datetimepicker.pas"
This appears to be a problem on both Linux and MacOS.
It appears to be a file permissions problem. When I changed the file permissions to "datetimepicker.pas", I got past that error, only to encounter another similar error. So when I upgraded permissions to my entire Lazarus installation, it now builds.
On MacOS, I did the following as root:
sh-3.2# cd /Applications/Lazarus/
sh-3.2# pwd
/Applications/Lazarus
sh-3.2# chmod -R 777 *
sh-3.2#
This is obviously not the best way to fix it, but if the admin people will look into this more, this at least appears to be the problem, and a workaround for it.
I hope this helps.