Novice errors while copying cells - advice required

Hello, everyone
I am completely new in macros/programming/VBA, but recently I have started to work on a project, which requires to write a macros to do the following:
1. From the active workbook choose the target workbook
2. Copy sheet from target workbook (without opening it) to active workbook as sheet2
3. Copy some values from sheet2 to sheet1 and perform some mathematical operations on them at the same time

Here is a piece of code i wrote:

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So the problem I am facing is that I can copy values using copy/paste commands, but this is not what I really want.
I'd like to copy values either using variables or assignment like this:

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But both options return me an error of 438:
Object doesn't support this property or method.
I am kindly asking to have a look a provide a piece of advice what I am doing wrong.
Thanks a lot in advance!

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Thank all of your for your replies!
I have changed my code as per the recommendations and it works now!