When using the Bogus Gateway, you want to ensure any other credit card payment providers are deactivated (Settings > Payments) first. Once you do so, you can enable the bogus gateway from the third-party gateway section in your Payment Settings and then activate it.
If you've already done that, what credit card info are you using when attempting to run a test transaction? The bogus gateway requires you to use a specific set of credit card numbers to simulate different types of checkouts. Using an authenticated credit card in the bogus gateway will not work. More on that can be found here.
If there is anything else I can help you with, please let me know.
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