I also got this error while installing your Roles plugin.
I made the following change (assuming role_id field can be a nullable field):
Changed the following line in the file:
After making the changes, used the octobercms backend to install the plugin, the problem got resolved.
Hello! You seems to be fixing the old plugin version. Because, firstly role_id column is renamed to vdomah_role_id according to new OctoberCMS plugin guidelines (from v1.3.0).
Secondly, the vdomah_role_id column was made nullable in v1.3.8 (Oct 09, 2020).