What's it mean when pokeman cannot be offered for trade 13267?

I don't know what the Number at the end of your question means.

On topic, it depends on the trade.

If you are trying to trade with an NPC then it means that the Pokemon you are offering is not what they want. For example, they could be asking for a female Dragonair and you might be offering a Voltorb.

If you are trying to trade with another person (NOT GTS), then they just don't like the Pokemon you are offering. They may be too demanding or you may be offering something not worth their Pokemon.

If you are trying to trade over the GTS, then you are either the problem is the same as the NPC scenario, or you are trying to trade a hacked Pokemon. Hacked Pokemon cannot be traded over the GTS (they can under certain circumstances, but they aren't common). Even if they could, very few people would want them. Their Pokemon was caught/trained/bred with sometimes hours or days of work and luck, and the hacked Pokemon took no effort. Besides, hacked Pokemon can damage save files, even if they were created on a different game and traded. The best thing to do is avoid them, but if you accidentally aquire one then the best thing to do is to release it before it can to too much damage.