Hi

Each LSA (Link State Advertisement) sent out by an OSPF speaking router needs to be acknowledged by a neighbour router with a Link State Acknowledgement packet.

When th router originates the LSA it starts a timer and if an acknowledgement is not received before the timer expires another LSA is sent out.

Your error message is saying that the router has declared it's neighbour down because it has sent too many retransmissions without receiving an acknowledgment.

If this adjancency was working before then

start troubleshooting this by checking the reliability of the link between the 2 routers then progress to the resources being used on the routers, is one too busy to respond etc.

HTH

Jon