This error message is due to the solution monitor (Velocity magnitude of mixture), which no longer have access to mixture velocity when you use Eulerian Model. When you use VOF model, velocity is shared between phases and you have the mixture velocity in the drop-down list.
So, after changing to the Eulerian model, you need to reset the monitors if you have set it already. If you check this monitor after changing the model, you can see that Phase and sub-field variable drop-down list is blank. Select appropriate field variable and the individual phases.