Latex error

[ZAFER ACAR <acarzfr@gmail.com>]

I get an error like this:

   1. This is BibTeX, Version 0.99d (TeX Live 2016/Debian)
   2. The top-level auxiliary file: acg_tez.aux
   3. The style file: plainnat.bst
   4. Database file #1: tez.bib
   5. Warning--I didn't find a database entry for "mts"
   6. Warning--I didn't find a database entry for "dav"
   7. (There were 2 warnings)


What can I do?