Any time you get a message that starts with 'Warning', you can be sure that it isn't an error. An error starts with the word 'Error', as one might expect.
In addition, the warning you got seems to be pretty clear? It says you already have DESeq2 installed, and that the version you have installed is the same as what you are trying to install, and if you really want to re-install you can use force = TRUE.
Can you say what you found confusing about that warning? Perhaps it could be worded differently?