Please refer to the screenshot, I have Member > Ecards relationship, I want to create a Relationship form but don't know why there is not field can be selected.
I would need a little more information to understand the nature of the issue,
1- Does this issue occur on all types of relationships? (One-to-one, one-to-many, many-to-many)
2- We had cases related to Firefox browser, could you please check with other browser just in case: https://toolset.com/forums/topic/relationship-forms-only-show-one-to-many-relationships-not-many-to-many-relationships/
3- Please make sure that WordPress, active theme, and plugins are all updated to the latest versions.
4- It would be interesting to test this with all non-Toolset plugins disabled and a default theme like Twenty Twenty-One.
If it's fixed, you can start adding the disabled items, one-by-one, to narrow down to a possible conflicting one.