« Ce n’est pas la mer à boire ».

Literal translation:
It’s not the sea to drink.

Meaning:
It’s not as bad as all that.
It’s not asking the impossible.
It’s not that big a deal.
It’s not that difficult.
It’s not rocket science.

Tu peux le faire. Ce n’est pas la mer à boire.
You can do it. It’s not that difficult.