Text was truncated or one or more characters had no match in the target code page including the primary key in an unpivot

Here is what fixed the problem for me. I did not have to convert to Excel. Just modified the DataType when choosing the data source to "text stream" (Figure 1). You can also check the "Edit Mappings" dialog to verify the change to the size (Figure 2).

Figure 1

Figure 2

I know this is an old question. The way I solved it - after failing by increasing the length or even changing to data type text - was creating an XLSX file and importing. It accurately detected the data type instead of setting all columns as varchar(50). Turns out nvarchar(255) for that column would have done it too.

I solved this problem by ORDERING my source data (xls, csv, whatever) such that the longest text values on at the top of the file. Excel is great. use the LEN() function on your challenging column. Order by that length value with the longest value on top of your dataset. Save. Try the import again.