Like in C# and Js Setting Lookup Value is different than setting String, It needs to information like entity type , name, and GUID.
Same way in power automate when setting up a lookup value or customer lookup we need to specify Entity and GUID or we can encounter error " Resource not found for the segment "

If we are getting below error most probably we are not setting Lookup field correctly

Resource not found for the segment ‘accoutid’


Empty segment encountered in request URL. Please make sure that a valid request URL is specified.

Below is the example , how to set Lookup field or customer field in Power automate flow

  1. if the field is a simple single lookup field, specify entity  name and GUID
    Example : contacts(GUID)
  2. If field is Customer lookup , Specify either account or contact
    Example : accounts(GUID) or contacts(GUID)