This generally occurs when your index pattern matches indices that have conflicting field types in their mappings. Can you take a look at your index pattern under management > Index Patterns and inspect the _path field? You should see a list of indices and the conflicting types similar to this screenshot:
This is from 7.2 so it might look slightly different for you depending on which version you're using. Once you've identified the index or indices which have the wrong mapping, you can fix this by reindexing the data into indices with the correct mappings defined. Here are some resources: