Hi Suzanne B,

I think, you are importing data in Dimension List.

Dimension list has property to keep atleast one unique property item in list.

If you have data, which do not have unique property than you can import your data in transaction list.

Hi Suzanne,

I hope you’re well! We have a short, free, training course which covers this, you can find it here: Create a Transactions List from an Import (gopigment.com)

Just to add some screenshots to Abhilashawlas answer, when you are on the ‘Map Data’ tab of the Import you will see a cog next to mapped dimensions

If you select this cog you can select which Unique Property of the Dimension will be imported into

You can identify Unique Properties in your Dimension by going on to it and seeing which Properties are underlined, for example ‘Name’ here;

If you right click on a Dimension Property and select ‘Edit Property’

You have the ability to make a Property unique.

Hope this helps!

Very clear thanks ! 
Indeed I had done that training, but had just forgotten to select transaction instead of dimension