A conflict usually occurs when two writers are offline and save a different revision of the same document.
As quick fix first get document from syn-gateway and
curl -X GET --header 'Accept: application/json' 'https://localhost:4985/uprofile%3Atesting/mydb?attachments=false&revs=false&show_exp=false'
Then get _rev value from document.. Then try delete as following
curl -X DELETE --header 'Accept: application/json' 'https://localhost:4985/uprofile%3Atesting/mydb?rev={_rev value}'
I am sure this will work,
but for permanent solution where to resolve conflicts dynamically check documentation https://developer.couchbase.com/documentation/mobile/current/guides/sync-gateway/resolving-conflicts/index.html