Open the Mixer (F9).
Make sure your bass is routed to one of the mixer channels, and your kick drum is routed to another channel.
If your kick and bass are still only in the Channel Rack, right-click each one's channel header and choose Assign to free mixer track.
In the Mixer, select the channel that carries your kick drum. Right-click the small routing arrow at the bottom of the bass channel strip and choose Sidechain to this track. This sends the kick to channel's sidechain input without mixing it into the bass channel’s main signal.
In the Mixer, select the channel that carries your bass (or bus of instruments you want to sidechain). Click an empty effect slot on that channel and add AutoChain from the plugin picker.
On the bass channel, open the AutoChain window. Click the Wrapper settings button in the top-left corner of the plugin panel (plug and gear icon), then open the Processing tab. Under Connections, enable 2. Sidechain and right click to select your kick channel from the dropdown. Close the wrapper settings to return to AutoChain.
AutoChain in FL Studio