I haven't seen them for the last one month.
It is still going on that I have not seen them the whole last month until now. That's why I use present perfect here. Am I right? Thanks.
Say "I haven't seen them...." (present perfect). That is because it is ongoing. I haven't seen them for a month and I still haven't seen them (up to the present).
Like Harry, I would say for a month.
I haven`t seen them for a month [ unfinished action- the process is still going on]
I didn`t see them last month.[ past action- finished]