Wireshark3.6.0 tcp.completeness field is a integer value to understand connection state.
the value consists of
1 : SYN
4 : ACK
8 : DATA
16 : FIN
32 : RST
when you see the value in your trace file at the Client side, it may be
1: just a SYN(1) packet, the firewall blocked the connection on server side.
3: SYN(1)+SYN/ACK(2), half connection, server is stressful or DoS attaked.
7: SYN(1)+SYN/ACK(2)+ACK(4) just a ESTABLISHED TCP connection without data


so Incomplete, DATA(15) means SYN(1)+SYN/ACK(2),+ACK(4)+DATA(8), TCP connection has been established and start data stream and not finished yet.