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’

or

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)