Make sure your data in Sheets is set to Plain Text and not Automatic (Highlight the column, select Format > Number > Plain Text. It will often format your column based on the first entry, which in your case may be Number. You’ll then need to Refresh Fields in your data source, and the type should switch to Text.
The other option is to create a data source calculated field in Looker Studio to force the value as text, like this CAST(Unit AS TEXT) and use that in your report instead of the original Unit field.