How do I fix "Could not connect to remote process" while trying to debug the application?

You need to restart the ADB.

kill adb.exe process.

how to restart ADB manually from Android Studio

Check if you have selected the right Build Variant, usually "release" comes with debuggable false

Example:

 buildTypes {
        release {
            debuggable false
            ....
        }
    }

is not recommendable change release to debuggable true, just change the right variant

   debug {
        debuggable true
        ....
    }

In My case, For some reason I disabled Instant run and it has changed the Launch value from "Default Activity" to "Nothing" in "Run/Debug configurations". So always it says "Could not connect to remote process. Aborting debug session" and I was not able to debug. I enabled Instant and changed the Launch value from "Nothing" to "Default Activity" in "Run/Debug configurations" and it worked. Hope this will help some one.

(to get Run/Debug configurations, click "app" drop down and click Edit Configurations)