I'm a bit clueless. How do I make an NPC available for roleplay in the sandbox? I mean, I've selected a profile for the NPC, then I've selected a profile for the player. But it tells me that I can’t start a chat with myself, or with a character who has the same profile as the player. Are there any specific settings needed? A guide or something like that would be nice.
I noticed that human models are not always detected when you click them (to focus them) when you open any UI window you will see in lower window area who is targeted character, so when I happen to get stuck in same problem as you just described, just click many times on model you want to target, and try to click different parts of that model, chest, face, legs, whatever... it will eventually detect what character you clicked, I'm sorry about that, but yes it is possible to get around it.
" But it tells me that I can’t start a chat with myself" if you did everything right, main character has Profile 1, and NPC character has Profile 2, and you get this error message, it just means that NPC character is not focused (clicked) aka the character you want to start chat with, so just click the NPC multiple times untill it shows in UI windows that it is selected.