Biblatex missing characters in Libertinus

[Wolfgang Engelmann]

Am 02.12.21 um 21:35 schrieb Herbert Voss:
> Wolfgang Engelmann schrieb:
>> ok, Herbert, you are ahead of me and the https://www.lyx.org/
>> since you are using lyx 2.4
>> where did you get it?
> 
> Ah yes. I had the development version.
> Now attached the version for 2.3.6 (original ubuntu)
> 
> Herbert
> 
As expected, Herberts test file does not work
The error message:

! Package fontspec Error: The font "STIX2Math" cannot be found.
For immediate help type H <return>.
  ...

l.13

A font might not be found for many reasons.
  Check the spelling, where the font is installed etc. etc.

  When in doubt, ask someone for help!

So I do, what the last sentence says!
-----------------------------------

What I did so far:
I do have the sticks2 fonts, as shown by
/usr/local/texlive/2021/texmf-dist/fonts$ find -iname 'stix2*'
May be, I have to set a PATH to it? How?
-----------
I put
  PATH=/usr/local/texlive/2021/bin/x86_64-linux:$PATH
in /etc/environment with sudo
The file was empty before. I rebootet and started my lyx file again, but 
get again
luaotfload | db : Reload initiated (formats: otf,ttf,ttc); reason: File 
not found: "STIX2Math.otf".
! Package fontspec Error: The font "STIX2Math" cannot be found.

I tried also /etc/environment.d
but I do not understand its functioning and stopped it
--------------
same after a reconfigure of the lyx file
-------------
my LyX version is 2.3.6
and
sudo apt-get install lyx
tells me for my version:
lyx ist schon die neueste Version (2.3.6-1).
0 aktualisiert, 0 neu installiert, 0 zu entfernen und 0 nicht aktualisiert.
-----------------
Thankful for any indication what I could try,

Wolfgang