Stop data entry if adjacent cell is blank - VBA code

Hello,

This may be easy for some of you but I am a bit stuck. I created a database table where users do data entry in excel.

What I am kindly requesting help on is the following: if a cell in Column A is blank, they shouldn't be able to input any data to the corresponding adjacent cell in Column B. I want to implement this constraint from A to AH. I have data from A to AH but a macro that will loop through all columns will be great. I know that this is possible through Data Validation but I already created drop-down lists for data validation for those columns. The only way I can do this seems like VBA.

Also I really appreciate if someone can help me out with putting specific criteria for some columns. Let's say if repair result in one column is failed, it is either scrap or rework. Likewise, all the data generated through drop down lists.

I tried the following but it doesn't seem like working:

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