How to use the same minmaxscaler used on the training data with new data?
You are refitting scaler_x on your test set, which you don't want.  Change this line:

xaa = scaler_x.fit_transform(xaa)


to

xaa = scaler_x.transform(xaa)


You are getting [-1, -1, ..., -1] because with one sample, each feature is equal to the minimum.