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:
I had the same problem. For my case, I went to pgAdmin>Schemas>Tables>blog_post and then arrow click to 'Colums' then found the colum, character varing(25). right click and then properties, click 'Definition. Finally I saw 'Length/Precision'. I changed 25 to 50. That's it. All good.
Once again, you need to change the size of your colum. go to pgAdmin>Schemas>Tables>(your app_model name)>Colums>(the colum you have problem)>Definition>Length
Change the size bigger. That's it!!!