I think Demerit's answer of adding org-to-individual should work. However, i'd thought i'd add some other considerations for anyone searching.
I've had this problem before, especially when using multi-client case setup.
I found that if there was a mismatch between any of the clients contact type/subtype with the new caseworker's contact type/subtype then i got this message.
This was further compounded by some users trying to add invalid contacts (e.g. a household) as clients to the case.
Steps to check
- Contact type/subtype of each client(s) involved in a case
- Contact type/subtype of the new case manager
- Check all combinations are valid
- If you're doing this with bulk import data there could be an issue missing data e.g. contact subtype.
I found the solution by
- reviewing relationship types Administer >> Option Lists >> Relationship Types
- Setting them to individual - individual (they were
the more narrow "contact sub-type A - contact sub-type B"). This lessend the chance of a conflict
- Amend the case open form or webform to limit the contact types that can be added as clients to limit only to valid contact types.