AppleScript Numbers Retreiving Cell Value
Here is a simplified example, showing minimal code, to replicate what's shown in your OP, and as you can see in the Replies pane of Script Editor in the image further below there are no errors and both display dialog commands returned the expected value. 

Numbers Document



Example AppleScript code:

set transferValue to 0

tell application "Numbers" to ¬
    tell table 1 of sheet 1 of document 1
        set transferValue to value of cell "B4"
        display dialog (get value of cell "B4")
        display dialog transferValue
    end tell 


Script Editor showing the results of running the code: