Q: How to build the rationale of the study if there is no research gap?
I have seen that its important to do a literature search to find the research gap (what has not been done). So, how I will build the rationale of my study if I find so many previous studies that have already been done?
Put emphasis on the fact that your replicating previous studies which is important for your research area.
You are adding to the body of evidence and you could comment on why you think this is important.
Perhaps consider providing extensive background detail on the context and built a case for the need to replicate the study. This will also pave the way to adding new to the knowledge that is unique to your context a s not visible in the previous study.
Another place could be to use alternative research approach (if possible) to see if change in approach may lead to different learning on the construct/ topic.