If you see the message Can't write license file then you need to make sure that you have moved MetashapePro application bundle from DMG archive to another folder, for example, to /Applications/.
DMG file is the archive that should be downloaded from the Installer page of agisoft.com website. Application bundle is what you see when you unpack DMG.
In the case of macOS Sierra, you may need to disable the Gatekeeper that is moving the application bundle to the temporary folder (AppTranslocation) without any write access rights. To do that it is necessary to execute the following command (with administrator privileges) from the Terminal, in some cases reboot may be required, after that, you should be able to activate Metashape Professional from GUI:
sudo spctl --master-disable
Alternatively, you can activate Metashape Professional from the command line (run Terminal from the Launchpad). In case you have moved the MetashapePro.app to /Applications/ folder, you should use the following command (for example, using the full path to the application) from the terminal using your license key as the last argument instead of the mock key used in the example below:
/Applications/MetashapePro.app/Contents/MacOS/MetashapePro --activate 11111-22222-33333-44444-55555