undefined method `async' (suckerpunch gem)

I am using sucker_punch gem to send the email in my rails app in the background.It used to work fine, but then I suddenly got this error:

undefined method `async' for #<ActiveJob::QueueAdapters::SuckerPunchAdapter::JobWrapper:0x007f892c6c31a0>

Below is the screenshot of the full error:

I am using sucker_punch version 2.0.1 right now. I have tried to downgrade it and using sucker_punch version 1.6.0 instead, but the error persist.

Any help? Thanks!