David is right,
i tested and the issue is that the new power apps UI is confused with the case of the entity name.
for example, try to create an new entity and name it "Test Entity" in the display name then save it.
now, even that the name (not display name) is clearley "***_testentity", you will get an error that "***_TestEntity" (note the upper case) is not found.
now try to create an entity and in display name enter "test entity 2" (all lower), save it then delete it it will be deleted without problems.
so you have 2 work arounds:
1-create the enttity with display name all lower case, then after saving it, open the entity, then click "settings" in the upper right corner and change back the display name to the correspongin case
2-use the classsic solution explorer:
- login to "https://web.powerapps.com"
- cllick "Solutions" on the left pane
- in the upper menu click "Switch to Classic"
- click "Common data services default solution" link in the list taht appears
- in the left pane, under "Components" expand "Entities". click the entity you want to delete. When it opens, in the upper menu click "Delete". Confirm you action and it should work
Hope this helps