This warning occurs when 'Repeat' flag which was supported earlier is used during UDF creation. %s - UDF name.
Remove the repeat flag from the code.
Example
[System: UDF]
MyUDF: String: 7678: Repeat
In the above example, repeat flag is used. Hence the message warning T0012: Repeat flag is not supported for the UDF '%s'.
The code is corrected by removing the Repeat flag.
[System: UDF]
MyUDF: String: 7678