After you create a Sponsored Brands campaign with a lifetime budget, you can increase the budget but you can’t decrease it.

A Sponsored Brands lifetime budget can be increased after campaign creation. It cannot be decreased once the campaign is active because Amazon Ads paces delivery against the total budget set for the campaign period. This helps prevent budget changes that could conflict with campaign pacing and delivery. Advertisers should choose the lifetime budget carefully when setting up a fixed-duration Sponsored Brands campaign.