BibTeX, biblatex and biber ⇒ Citation undefined warning

[hasnain]

Hi,
I am farely new to Latex and have some issues in inserting bibliography,
I am getting these type of errors for all of my citations in my latex file, "Package natbib Warning: Citation `Bergmann2006' on page 93 undefined on input line 6."
I have also attached my .bib file.
My console seems stuck here whenever i run References.bib file separately.

This is pdfTeX, Version 3.141592653-2.6-1.40.25 (MiKTeX 23.4) (preloaded format=pdftex.fmt)
restricted \write18 enabled.
entering extended mode
(D:/Thesis/Tomography/Report/References.bib)
*

Also weirdly i see all my citations whenever i write \cite{} Command but the it still for some reason won't generate the bibliography.

Additionally I have added apacite as bibliography style and package,

Thanks in advance for guidance in this regard,

[rais]

My console seems stuck here whenever i run References.bib file separately.

Whatever gave you that idea? The .bib file is not directly processed---except by your text editor.
Say, your main .tex file (the one containing \documentclass) is called foo.tex, the normal `incantation' looks like this:

1. LaTeX foo[.tex]
2. bibtex foo[.aux]
3. LaTeX foo[.tex]
4. LaTeX foo[.tex]

where LaTeX stands in for either latex, pdflatex, xelatex, lualatex, whatever you normally use.
The first LaTeX run causes all necessary data for bibtex's work to be written into foo.aux.
Bibtex then creates foo.bbl out of the data garnered from the foo.aux file.
The second LaTeX run then loads the aforementioned foo.bbl file (that's part of the \bibliography command).
The third (and possibly fourth) LaTeX run just cleans up labels that may have changed.
The file extensions can be omitted for all programs shown here, hence I put them in brackets, e.g., instead of running, say `pdflatex foo.tex', you might as well use `pdflatex foo'.

BTW: for new documents I'd rather use biblatex/biber (where the above bibtex line changes to `biber foo[.bcf]', but then, that wouldn't be the only change...)

KR
Rainer