Why did I receive an error when I convert a lead that has more than 255 characters in a custom Description field?

You can't convert a lead if your custom Description field has exceeded the maximum limit of 255 characters.

The long text Description custom field must correspond to the Description field character limit values that are copied from the lead to the account during the conversion process. Reduce the number of characters by less than or equal to 255 characters and try again. Alternatively, request that your administrator increase or remove the character constraint for the custom Description field in Leads.