In some cases, route solver does not solve and returns an error message when the U-Turn policy is set at 'Only At Dead Ends' and the 'Reorder Stops To Find Optimal Route' option is enabled in the Network Analysis extension in ArcMap 10.6.
1. After getting the error, take note of the sequence of stops that the Network Analyst uses.2. Disable the 'Reorder Stops To Find Optimal Route' option and copy the sequence before solving.