execution reverted: Leaf index is too big - What does any of this mean please??

execution reverted: Leaf index is too big - What does any of this mean please??

Have tried to send a small amount of Matic back to the ETH mainnet in Metamask. There's enough ETH for gas fees in Metamask and I left some Matic behind for the gas fee there too. Am at checkpoint stage, but nothing seems to work from here. Someone, please help...
Red Execution Reverted message, but coins are gone...