what does [citation] mean in google scholar search results?

What does [citation] mean in Google scholar search results and why is it not clickable? I know there is also an option to include citations but I am not sure what that means? If something is cited is should exist somewhere.

example:

[CITATION] Use of detergents in the analysis of fibrous feeds. IV. Determination of plant cell-wall constituents PJ Van Soest, RH Wine - J. Assoc. Off. Anal. Chem, 1967