You cannot define an export that will be a fixed width from SSRS - in order for a fixed width file to be generated you need to know the length of every field and pad every field to that length.
This cannot be defined as a possible export because that information will not be known at execution.
If you must use SSRS to export fixed width files then you need to do that in SQL. Depending on the structure of the output - you may need to include additional columns for sorting the data appropriately, but then you will display the single formatted column.
Now - you need to add a custom rendering for a text file, see this article: https://www.asr-solutions.com/2018/04/12/ssrs-custom-rendering-for-output-text-files/
With that, the user can then export to a text file and the report data will be fixed width because each row is already formatted as fixed width.
Or...you can create a single column in the report as an expression and build each field, padding out to the required length for each defined field - but that would be much harder to manage and maintain than doing it in SQL.