By specifying the column as VARCHAR(500) you've set an explicit 500 character limit. You might not have done this yourself explicitly, but Django has done it for you somewhere. Telling you where is hard when you haven't shown your model, the full error text, or the query that produced the error.

If you don't want one, use an unqualified VARCHAR, or use the TEXT type.

varchar and text are limited in length only by the system limits on column size - about 1GB - and by your memory. However, adding a length-qualifier to varchar sets a smaller limit manually. All of the following are largely equivalent:

column_name VARCHAR(500)

column_name VARCHAR CHECK (length(column_name) <= 500) 

column_name TEXT CHECK (length(column_name) <= 500)