34000 and 52000 indicate the natural form of these elements. That is, it is asking MCNP to look for the natural isotope distribution library for Se and Te. Several of the natural substances are not there while the individual isotopes are there.
Depending what is in your library, you have some choices. If the individual isotopes are there, you could build your material out of them. So, Se for example:
https://en.wikipedia.org/wiki/Selenium
You could look in your lib for 34074. 34076, 34077, 34078, 34080, and 34082. If they are all there you can build natural Se out of them. Pretty close, anyway.
If those isotopes are not in your library, you can try to find a material that you can substitute. That's a fairly complicated thing. And it's difficult to be confident if you don't have any data to guide you. Maybe if only some of those isotopes are present, you could build a partial model, maybe using the more common isotopes.
If you are keen, and need these isotopes, and you have the time, you could see if the data exists somewhere on line. Then you could learn the isotope library utils that come with MCNP. NJOY is one of them.