This message appears when you try to change the budget of a campaign that doesn't use a campaign-level budget. Meta has two models: CBO, where the budget sits at the campaign level and is distributed across ad sets, and ABO, where each ad set has its own budget. In ABO campaigns, the Budget column shows an ABO label instead of an amount, because there is no budget to edit there. To change it, go to the Ad Sets tab, select the campaign's ad sets, and edit the budget there. This is a Meta rule, not an Ads Editor limitation.