I'm using the major push upgrades to install a new version of my managed package in our customer.
In this package I have a new field and it didn't exist in an older version of the package. In the postInstall script I'm trying to populate this field. Then I got the error: "Field is not writeable".
How can I solve it? I already setted a permission set for all the users giving access to the field, but the postInstall script is executed with another user.