With the error description its quiet evident that the External tables does not support row size more than 32768 bytes. But still I take a look online and found in Azure Documentation that this is a limitation right now with Polybase. The Azure document mentions:
Wide rows support is not supported yet, “If you are using Polybase to load your tables, define your tables so that the maximum possible row size, including the full length of variable length columns, does not exceed 32,767 bytes. While you can define a row with variable length data that can exceed this figure, and load rows with BCP, you will not be be able to use Polybase to load this data quite yet. Polybase support for wide rows will be added soon. Also, try to limit the size of your variable length columns for even better throughput for running queries.”
You can still use varchar(max) and nvarchar(max) for data types (unlike the Hive provider, which has a strict limit of 8000 characters for a single column) but can’t break that 32K mark.