As @sureshprasanna pointed, the template used for making API request might be missing or wrong syntax. Filter API requires the filter params to be URL encoded. It should be URL encoded before using it in the request. Postman will automatically do.
So, could you share the request template used so that we can point out what might be wrong?
Also, could you enter the entire URL along with query params URL encoded passed in the Path in the request template and make it work? After it works, then whatever needs to be passed dynamically can be passed from iparams and context templates.
Some solutions available in the community: