I think there is a little grammatical error in the statement. The statement should be “An SR sender’s window base cannot have a lower sequence number than the receiver window base”.
Honestly, the statement “An SR sender’s window base cannot have a lower sequence number than the receiver window base” does not make any sense.” Because let’s say we have packets with sequence numbers 0,1,2,3,4 and somehow the packet with sequence number ‘0’ is lost or corrupted, as per the functioning of SR, it will be re-transmitting the packet with sequence number ‘0’, which is a lower sequence number.
Suppose we have received an ACK for the acknowledged packet, so the thing that happens in SR is that the sender window base is moved forward to the unacknowledged packet with the smallest sequence number. If the window
moves and there are untransmitted packets with sequence numbers that now
fall within the window, these packets are transmitted.
So that’s why the statement is false. If there is any other reason than this, do share.
Ref: Selective-Repeat (Page No. 3 -> Figure No. 3 -> Point No. 3)
Ref: CN – 6th ed Kurose & Ross (Page no. 225 & 226)