Error: "Cannot update bid for this targeting clause when the ad group has an enabled optimization rule."

As you may know, Amazon can be weird sometimes.

They recently introduced the "Cost Control (Beta)" feature for Sponsored Display:

When this optimization rule is enabled, Amazon prohibits manual bid adjustments for all targets within that campaign.

It's currently not possible to disable this rule. So the only way to truly turn it "off" is to create a new campaign without this feature enabled.