⟨ERROR⟩ attribute layout_constraintBottom_toBottomOf... Not found

[jason-j-neuhoff]

I have difficulty navigating around this problem.

I added this attribute within the wrapper in my "activity_main.xml" file.

Though technically I found out that you should only use that attribute inside the ConstraintLayout. And it is possible to use it inside of the FrameLayout and then android:gravity="bottom" in my linear layout, but that is too much of a fuss.

So the solution would be to add this line into my app/build.gradle under dependencies
↓↓↓
implementation 'com.android.support.constraint:constraint-layout:1.1.3'

Though the problem I face is that the app doesn't support adding app dependencies. And that it suggest I try using "the library manager instead". But how do I use the library manager? Where can I find it?
Is it possible to rename the .gradle to a .txt and adding that line and converting it back?

Help please, as my project is at a standstill atm.