Is the problem with anarchists that they cannot stay together?

In the case of the Spanish civil war, the problem was that the communists and the fascists both wanted them killed, and so with some effort they succeeded.

Anarchists also attacked communists when it suited them. https://es.wikipedia.org/wiki/Antonio_Ses%C3%A9

Any documentary evidence that elucidates why the communists were against the anarchists?

Loads and loads of books, historical documents, documentaries , and so on on the matter.

Not just in Spain, in USSR and elsewhere too.

In fact the ideological enmity between communists and anarchists starts even back at the time of Bakhunin and Marx (and they were many critiques from one to another).

Yes, in the first years of the Federación Regional: https://es.wikipedia.org/wiki/Federaci%C3%B3n_Regional_Espa%... in fact from their separation the socialist party (PSOE, the second most voted party in SPain nowadays) was created by Pablo Iglesias.

Also see the history of anarchist "Free Territory" in present-day Ukraine. https://en.wikipedia.org/wiki/Free_Territory

The strategy of the Stalin-backed communists was to not alienate the middle class. The anarchists, on the other hand, were not about letting war get in the way if social reform, and had taken over factories and collectivised land. It was also a good opportunity for Stalin to purge communists not allied with the USSR.

In short: communists generally wanted a central government/(controlled by he socialist party). And anarchists not.

Well its a marxist magazine, so its essentially pushing a political line that the anarchists were wrong not to pursuing socialist goals.