If your agent is able to put a caller on park when it is a direct or internal call but NOT when it is a skill group call, it is likely the IP Office PBX is treating the virtual handset making the transfer as a IPOCC handset which does not allow this feature by default.
To correct this issue, you'll need to make a modification in IPO Manager.
1) In Manager, navigate to Users and find the default user named "NoUser"
2) In the configuration for NoUser navigate to the Source Numbers tab
3) Add a new source number with the code of ACCS_FEATUREMASK=00000005
4) Save and Merge your changes in IPO
This will override the IPOCC default that is blocking the park event.