I attempted to recreate the issue. The only way I could get the OK button to be greyed out was to select a target project which is not editable and to select “Automatically export verses when approved”. So, my guess is Paratext thinks that the target project is for some reason not editable.
So if you have already done send/receive, you could try deleting the target project and send/receive it again to see if anything changes. Otherwise, my next guess would be a permission problem, since occasionally there is a disconnect between one computer’s permissions and what the PT server or another computer says.