I can't find homepage SEF URL

It's a Joomla Core (mod_mainmenu) related issue, there are 2 ways to fix it:

a. Create the SEF URL of homepage manually

1. Go to menu that your Default menu item is pointed (generally main menu)
2. Click on the menu item and copy the ID and Link entries then click on Cancel to get out of there
3. Finalize the Real URL by uniting the copied parts like: index.php?option=com_content&view=frontpage&Itemid=1
4. Go to AceSEF=>SEF URLs, click on New button
5. Leave blank the SEF URL field and enter the created Real URL to the Real URL field, then set other parts as you want (meta etc)

b. Fix the mainmenu module

OPEN

modules/mod_mainmenu/helper.php

FIND

$tmp->url = JURI::base();

REPLACE WITH

$tmp->url = str_replace(array($tmp->route.'/',
$tmp->route), '', JRoute::_($tmp->url));