No, JIMCHI; if you do not have a Household Account (often abbreviated here to HHA) which 'links' family members' individual Executive Club accounts, then you are free to spend your miles on whoever you like.

Accordingly, you will simply follow the process that you've already tried, and be successful.

Had you been part of an HHA, then you would have been limited to selecting the names of registered members of the HHA from a list with radio-buttons next to them.

No cause for alarm. Carry on.