This error often appears when you are using a specific type of bus in the "Bus Creator" block. You can check this setting in "Output Data Type" of its properties dialog. For example, in the image below, the "Bus Creator" uses a custom bus named "MyBus":
The most common occurrence of this error is when the number of inputs to the "Bus Creator" block do not equal the number of elements in the specified bus type.
Additionally, it can also appear when there is a mismatch between the input signal names and the bus elements' signal names. This is the case asked in the question. This is related to the setting "Override bus signal names from inputs", and unchecking it should resolve the issue.
The "Override bus signal names from inputs" setting causes the names of the incoming signals to determine the names of the elements of the bus. Even if the incoming signals do not have names, the default names of "signal1", "signal2", etc. are used. This creates a problem if you try to set the output data type to be a specific type of bus with specific element names. The names end up not matching and you receive the error message regarding the number of signals not matching.
