I followed your installation instructions with REDCap version 6.15.14
On the user interface, REDCap doesn't mention any error when sending an e-mail but it never reaches destination. Without receiving emails, it was impossible to set a password for the first user.
In the /var/log/mail.log I had the following error :
postfix/postdrop[]: warning: unable to look up public/pickup: No such file or directory
Googling it, I found this solution :
mkfifo /var/spool/postfix/public/pickup
service postfix restart
It worked.