The issue is due to the dot (.) sign in your column name.

1. To use column mapping, you should also specify structure in your source and sink dataset.
2. For your source dataset, you need specify your format correctly. And since your column name has dot, you need specify the json path as following.
3. You could use ADF UI to setup a copy for a single file first to get the related format, structure and column mapping format. Then change it to lookup.

The issue is due to the dot (.) sign in your column name.

1. To use column mapping, you should also specify structure in your source and sink dataset.
2. For your source dataset, you need specify your format correctly. And since your column name has dot, you need specify the json path as following.
3. You could use ADF UI to setup a copy for a single file first to get the related format, structure and column mapping format. Then change it to lookup.

And as my understanding, your first format should be the right format. If it is already in json format, then you may not need use "json" function in your expression.