You need Ripgrep installed in your machine. And it needs to be compiled with pcre2 option (I have no idea what it does, but it is required).
If you use Ubuntu (of some recent version), avoid apt because the binary does not seem to have been compiled with this option. I suggest compile from source.
If you are on Windows, I have not been able to get it to work, even when the binary comes with prec2 option. This seems to be related to the fact how the output from Ripgrep gets converted to unlinked references in Org-roam – to fix this, I think you’d need to change the function in Org-roam (only for Windows, by the looks of it). I didn’t investigate any further.
Some detail from this old post. It’s old, so the situation might well have improved / changed since then.