I had tried to create a Data table with fields Name and Gender as dropdown list and tried the use case as shown in attachments.
Can you please look into it and confirm me in detail what is your exact requirement.
Branch Code field might be the drop down list in Data Table which has multiple options to choose while creation of records into Data table, but in the report definition what is the need of having drop down as it only holds a single value, by default it is PromtSelect control is being selected in report definition as shown in attachment.
Kindly let me know the exact usecase you have at your end to understand more.
The fields BankCode/BranchName/.. all are Text(single line) type then user can only enter a single value in this field, but for the dropdown field you should have the multiple values. So it was not clear about your requirement where you want the drop down field and what list data you want to display in it.
As per our screen share, I had clarified the your requirement that you have got confused in order to change the single value column into the drop down list which is not possible as it only holds the single value. If you want to add a drop down list the field should hold the list of data in it.
So I suggested you to fetch the list of column data using the sub report and use the result into the corresponding column where you need the drop down list.