ADB server version (36) doesn't match this client (39) {Not using Genymotion}

This works for me...

• go to GenyMotion settings -> ADB tab
• instead of Use Genymotion Android tools, choose custom Android SDK Tools and then browse your installed SDK.

In my case this error occured when I set up my environment adb path as ~/.android-sdk/platform-tools (which happens when e.g. android-platform-tools is installed via homebrew), which version was 36, but Android Studio project has Android SDK next path ~/Library/Android/sdk which adb version was 39.

I have changed my PATH to platform-tools to ~/Library/Android/sdk/platform-tools and error was solved

First of all, please remove the "{Not using Genymotion}" from the title. It distracts readers like me who don't know what Genymotion is. The absurd here is that you got the second highest voted answer with currently 90 points which says "go to GenyMotion settings"...

The main point that all the others have missed, is that you will get this error when you have a running adb process in the background. So the first step is to find it and kill it:

ps aux | grep adb
user          46803   0.0  0.0  2442020    816 s023  S+    5:07AM   0:00.00 grep adb
user          46636   0.0  0.0   651740   3084   ??  S     5:07AM   0:00.02 adb -P 5037 fork-server server

When you find it, you can kill it using kill -9 46636.

In my case, the problem was an old version of adb coming from GapDebug. If you got this with GapDebug, get out of it and then do

adb kill-server
adb start-server

because with GapDebug in the background, when you kill the adb server, GapDebug will start its own copy immediately, causing the start-server to be ignored