Drupal in D8 added uuidUUID to identify & differentiate one Drupal from another and was also added so people don't accidentally add config to the wrong site.
So you already got an export from site A.
Now on site B do an export for an existing webform, like Contact (which was created by default when you installed webform) and copy its UUID value.
Now use this UUID and replace site A's export UUID with it.
Important
If this is a one time thing, the above is alright. But if you're going to constantly export config from site A to site B multiple times, then perhaps chaging site's B UUID to A's (so they are identical) is the better solution, see How can I import the configuration on a different site?