django JSONField doesn't accept value

This is not valid JSON. The strings in JSON are wrapped with double quotes. Indeed, you can verify this for example with JSONLint.com. It is a valid Python dictionary, but not valid JSON.

You thus should enter:

{ "key": "value" }

For more information, see the JSON specifications.

I add products in admin panel. I want to have multiple attributes for my products like color , size and ... . is it true to use json fiels for that??

If the fields are fixed (are attributes of all Products), then you should use individual fields, since that is more convenient to fielter, make forms, etc. If the data is more dynamic, it is often better to make use of a JSON field.