The rack database model has changed in Netbox 2.7.
Therefore the initializer code, that was used before, broke.
As a user, you will need to update your racks.yml file as follows:

- Rack types must match one of the 5 rack types given, e.g.
  '4-post-cabinet'.
- Rack width must match one of the 2 rack widths given, i.e. '19' or
  '23'.

See the diff of this commit for further information how this is meant.