I think you have already been pointed to
but let me point you there again - you are much more likely to receive help here if you provide people with runnable code that reproduces your error. In this example here, your error is produced by a call to make_minibatch(x_test, y_test), but both x_test and y_test are undefined.
Also make sure to use triple backticks ``` to make your code legible.
The error your getting should be reasonably obvious though - you are trying to convert a value of type RGB{N0f8} to Float32, and the error is telling you that no method doing this is defined. I don’t know much about image processing but I’m not sure what you would expect the result of a conversion of an RGB object (which presumably consists of three numbers) to Float32 to be.
Maybe this SO answer is relevant: