Method 1 If you want to use the Mann-Whitney Test tool (Statistics: Nonparametric Tests: Mann-Whitney Test), you need to reorganize your data as the following image shows. And select col(B) as Group Range, col(A) as Data Range after you opened the tool.
Method 2 If you don't want to change your datasets' organization, you can change to use the (NPH) Two Independent Samples tool (Statistics: Nonparametric Tests: (NPH) Two Independent Samples). And set the dialog as the image shows