I can’t add a link from one custom object to another.

Description description

Creating a many-to-many or N:N custom object structure such that a lead or company can be associated with multiple custom objects and a custom object can be associated with multiple leads or companies at the same time. When adding a link to the secondary custom object, that secondary object is not list as an available linked object.

Resolution resolution

If you find that the custom object you’d like to link to is not list in the “Link Object” list, there are two things two check:

Since it’s not possible to change a link field once a custom object is approved, if you already have link fields, the only solution is discard the secondary custom object and create a new one. Recreate the custom object without link fields. Then, in the intermediary object, the target object will show up as an available link object.

More detail can be found at:

Root Cause:
The secondary custom object type is not approved.