Error :
The directory that you created for Elgg to save files (the permissions on this directory are checked when you click Next). It must be an absolute path.
How To Fix ?
1) Make dir for example : ( /var/www/dataelgg ).
Debian/Ubuntu Command :
mkdir /var/www/dataelgg
2) Give Full Permissions. ( 777 ).
Debian/Ubuntu Command :
chmod 777 /var/www/dataelgg