It is no longer possible to enter both an amount and a percentage or an equivalence number in the one settlement rule. This has been so since release 4.0A. Please follow the long text of the eror message and correct your settlement rule as appropriate.
Since release 3.0A settlement by amount was made possible. The amount is settled first. The percentage is applied to the rest.
Some information in regards to deleting distribution rules/settlemt rules if need be. It is possible. You go into AIAB and either delete and recreate or change existing rules. If you cannot delete it via AIAB, you maybe can do so via direct table corrections - although do consider with extrem care. In that case I strongly recommend you should discuss this further with a consultant in order to not create any inconsistencies, but also in case you don't have the necessary experience. I think the relevant table would be COBRB. However, this should be reviewed very carefully before this approach is taken as it is never recommended to do a direct table change and always should only be considered if no other option available and after discussing with an expert consultant.
I hope this information helps you however.