This is the 17905th most frequent Polish word.
łańcuchu
"Łańcuchu" means "chain" (locative or dative case).
In this sentence, "łańcuchu" refers to the chain of the bicycle, used in the locative case.
Rower naprawiono, ale nadal coś jest nie tak z łańcuchu.
The bicycle has been repaired, but something is still wrong with the chain.
Here, "łańcuchu" indicates the chain, suggesting a literal chain that the key might unlock, used in the locative case.
Trzymam klucz w ręku, ale nie mogę go użyć na łańcuchu.
I am holding the key in my hand, but I cannot use it on the chain.
In this example, "łańcuchu" metaphorically describes the chain of command, used in the locative case.