I’m not sure how to use Predict[]

In my university I got an assignment: I need to predict the stock price using two of machine learning methods with imported data. (I’ve chosen Linear regression and Decision tree).
I tried to make a program myself but encountered errors and since I’m a rookie in machine learning I don’t quite understand what I’m doing wrong.
I added a screenshot how the data in imported file looks like just in case.
So here’s my code:
a = Rest[Import["Test.xlsx", {"Data"}][[1]]][[;; - 2, {2, 3, 4, 5, 6, 7}]];
b = Rest[Import["Test.xlsx", {"Data"}][[1]]][[;; - 2, {8}]];
trainingset = {a -> b};
prediction1 = Predict[trainingset, Method -> "LinearRegression"];
prediction2 = Predict[trainingset, Method -> "DecisionTree"];
At lines where I use Predict[] I got these error:
"Predict: Incompatible variable type (Numerical) and variable value ({{243.73},{242.65},{240.82},{240.87},{239.1},{236.1},{234.5},{238.18},{237.6},{238.91},<<991>>})."
So can someone explain me what’s going on here?

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