We have started work to remove this restriction.
Current ETA is for second half of 2020
An error occurred while saving the commentDerek Davison commented
Just ran into this myself whilst attempting to create a Linux App Service Plan in an existing RG with tons of other Windows resources. The error I got after clicking "Create" in the Portal wasn't very helpful either. All it said was:
"Requested feature is not available in resource group <MyRG>. Please try using a different resource group or create a new one."
Would be nice to either allow it or update the message to relay the real issue that Linux and Windows App Services cannot exist on the same Service Plan.