Description of the error shown when trying to run a simulation: A multi-region mesh was assigned - this analysis-type requires a single-region mesh.
These may be the causes (extracted from: Error: Multi-Region Mesh in Non-CHT Simulation | SimScale)
A second common cause is related to the Advanced concepts . For single-region analysis, we may intentionally have more volumes when we are modeling advanced concepts:
If the user forgets to create a cell zone and assign it to an advanced concept, it will result in a multi-region mesh error when running a simulation.