Where is the ACK to the packet in frame 76? [closed]

The acknowledgement number in a segment with the ACK flag acknowledges everything prior to the acknowledgement number. There does not need to a an ACK segment corresponding to each segment sent. You can have sporadic acknowledgements that encompass several segments.

If the receive window is large enough that multiple segments can be sent, then acknowledgements need only come for multiple segments sent. For example, if the receive window is large enough for 10 segments, you could have an acknowledgment after every sent segment, or every tenth segment on any combination in between, and it does not need to be regular, such as after four segments, then seven more, then 2 more, etc.