Let's say your whole model is executed at every 0.1s. At every 0.1 second, the condition is checked and the "If-action" subsystem is executed or not executed. So the execution of the subsystem is asynchronous or triggered. I can't think of a scenerio where inside the subsystem, a rate transition is meaningful, although your error message says there might be such a case.
Move the rate transiton block outside of the "if-action" subsystem. I can't imagine why you can't.
Added below:
Now I think I might understand such a case indicated in the error message. When the "If-action" subsystem is executed, all its inputs must be available. The "If-action" subsystem is only executed at most once per step size, so there is no need to do rate transition inside. However, if one of its input is asynchronous or triggered which means it is not available at the time when the "If-action" subsystem is executed, then you might need to do rate transition inside (for the purpose of holding last available value). Still, even in that case, I think it is possible or better to do the rate transition outside.
Best Answer