I use joomsef, and want to set up urls like /domain.com/ttfm that will redirect to a page I have about that topic.

but in the management/SEF url setup, I get the message: must start with index.php

The thing is that I already run Joomsef, and can not turn off the JoomSEF to find the non-sef url.

Why cant I type, pick url from browsing my site and insert, then save and that way create my desired sef url 301 redirect...

Hi,

You can't create a SEF URL for this, you need to create a 301 Redirect. Open the Manage 301 Redirects page and create your redirect there.
Please read the documentation for more information about 301 redirects at: www.artio.net/joomsef/documentation#d4e1514

understood and done. Thanks.