In looking at the screen shot where the values are shown on the bottom of the screen, the ID for the User node has a '55' in it, but the value being used in the match query has a '54' in it. It appears the values being used in the query to do the deletion don't match the values in the database.
I tried this example and it worked fine. First creating the data values to delete:
create (u:User {id:'95e54ae5-e512-4030-9640-6b157aaf5400'})
create (p:Post {id:'919f3216-b13b-48ae-a4df-ef779325ff5e'})
merge (u)-[:LIKES]->(p)
That created the nodes and the relationship.
Added 2 labels, created 2 nodes, set 2 properties, created 1 relationship, completed after 2 ms.
Then they deleted fine using the initial query that was posted:
MATCH (u:User)-[r:LIKES]-(p:Post)
WHERE u.id="95e54ae5-e512-4030-9640-6b157aaf5400" AND p.id="919f3216-b13b-48ae-a4df-ef779325ff5e"
DELETE r
With the results:
Deleted 1 relationship, completed after 2 ms.
Double check the values being passed in as the IDs. On the screen shots were both the query values and database values are shown, the values in the query do not match the values in the database.