Deals with gold shows Diablo 3 on sale but it won’t let me purchase the sale

That was part if last week's deals with gold. Deals with gold games change every Tuesday so it would not longer be on sale anymore and should not have shown on sale when you checked earlier. Once a sale is over, it is not possible to purchase a game at that
price unless it goes on sale again. You can keep an eye on sales at majornelson.com.

That is not a mistake. If you would like to take advantage of the deal you would need to purchase the game and season pass separately.

majornelson.com/.../deals-with-gold-and-ultimate-game-sale

Deals with Gold change every Tuesday. Bioshock was part of last week's deals with gold so it is no longer on sale. https://majornelson.com/2017/10/02/this-weeks-deals-with-gold-and-spotlight-sale-100/
There is no way to get the sale price again unless it goes on sale again.

The Surge is not currently on sale - https://majornelson.com/2018/04/23/this-weeks-deals-with-gold-and-spotlight-sale-curve-digital-publi...

It was part of last week's Deals with Gold.

According to Major Nelson's page the deal was only until February 29.

majornelson.com/.../this-weeks-deals-with-gold-and-spotlight-sale-40

If it is showing as the deal price for you still then I would recommend taking advantage of it as the price will change later today.