First things first!
Please ensure you have the correct version of Amplitude for your version of Unity. Unity has made some breaking changes over the past couple of years and the breaks occur mid-stream within their branches. Please see the chart below. If you are importing Amplitude into a new project it is easy to ensure you have the correct version. Always ensure you Update (if available) before you import. It is a good idea to Re-Download to ensure you have the correct version as well, then Import.
Next, confirm you have the correct version -- examine the readme file and compare to the chart below.
Unity version and Amplitude build requirements chart:
Now, on with the show!