The world is an unpredictable place - wars break out, there is injustice and corruption.
You cannot predict, still less control, the weather, but there comes a time when you must make your decision and act. If you wait until you are sure of the outcome, you will wait forever and lose everything. The right response to uncertainty is not to do nothing, but to factor the uncertainty into your planning. You cannot assume that everything you do will succeed - some things will not succeed - but if you spread your risks, that may bring gain in the long term.
Everything is subject to uncertainty. There is no time, and no climate, so favorable that you can be certain of success if you sow your seed then. There are limits on how much gain you can reasonably expect to make.
The Teacher goes behind the individual uncertainties of life to the most fundamental uncertainty of all - we don't know what God is doing.
God is the Maker of all things - everything that happens ultimately comes from God. Precisely for that reason we must acknowledge that we cannot understand the work of God. We can see (or think we can see) the reasons for some things that happen, but none of us can explain everything that happens in God's world.
All our ventures, then, are subject to the caveat - God Willing.
We must make decisions, plan as best we can and go for it. But we must also expect the unexpected and reckon with life's uncertainties.
As The Teacher has often reminded us, our knowledge of God's ways is limited, and to accept this limitation - unwelcome though it may be - is the way of wisdom.
After all, if it is a choice between seeing God's purposes fulfilled or our own plans succeed, surely we would prefer the former.