Hi @boriskamp1991
I highly suggest that you debug your code line by line:
http://www.advancedcustomfields.com/resources/how-to/debug/
It is most likely that the $target_product variable is not actually a single post object, but an array of them. This would explain the error.
If you have trouble identifying the issue from your debugging, please post back the result of each variable and i”ll help point you in the right direction
Cheers
E
