Hello
This teleconverter is not compatible with the 70-300mm. The lens must have a special recess. You can see which lenses are compatible under this link:
https://support.d-imaging.sony.co.jp/www...
However, please note that the AF-C on the A7iii is only supported up to aperture F11. With the 200-600 with a maximum aperture of F6.3 at 600mm, this means an aperture of 13 with the 2x teleconverter and a correspondingly high ISO.
If you want to use a teleconverter, fast F2.8 lenses like the 70-200 f2.8 GMaster are recommended. If you can do without 2.8, I would recommend the 100-400 GMaster or, if you have the financial means, a combination of 70-200 f2.8 GMaster and 200-600mm f5.6-6.3 G.
