Since inviscid flow problems will usually involve high-speed flow, you may have to reduce the under-relaxation factors for momentum (if you are using the pressure-based solver) or reduce the Courant number (if you are using the density-based solver), in order to get the solution started. Once the flow is started and the residuals are monotonically decreasing, you can start increasing the under-relaxation factors or Courant number back up to the default values.
Modifications to the under-relaxation factors and the Courant number can be made in the Solution Controls panel.
Solve Controls Solution...
The solution strategies for compressible flows apply also to inviscid flows. See Section 9.6.5 for details.