Magento 2 Oauth error invalid signature while try to get request token from postman
If you supply all of those values, you don't have to use the access or request routes. postman will take care of the rest.
Typically I've noticed that some routes are slightly different; ie.
use: http://your url here/rest/V1/products?searchCriteria=
instead of: http://your url here/rest/V1/products?searchCriteria
Notice the difference with the =
The only time you need to use those routes are when you post to your callback and identity urls.