The button is shown in creative mode.

Once the player "discovers" at least one of the 12 runes that make up the signature for the travel journal entry, then the tab will appear in survival.

This probably isn't super clear in the documentation so I'll update it there.

Once the player "discovers" at least one of the 12 runes that make up the signature for the travel journal entry, then the tab will appear in survival.

This probably isn't super clear in the documentation so I'll update it there.

So what can be qualified as "discover"? Craft using stone cutter or just have them inside inventory? I tried to craft them with polished amethyst but still I can neither see the travel journal tab nor enter the portal.

It's when you throw an ender pearl through a runestone. There are 26 runestones, each one corresponding to one "letter" of the standard galactic alphabet.
So let's say you throw ender pearl through runestone "A", your player then "discovers" runestone "A" and if the rune portal for a travel journal entry contains this rune, it will appear instead of a "?".

I think I'll just make it so that the rune button is always enabled.
This means that the rune portal diagram will show as "?" if a rune hasn't been travelled by the player.