If I put money into Roth IRA until a certain age, will it devalue?

If you look at the long-term chart of the DJIA, you will see that it's had its ups and downs over the years although the trend has been upwards. Theoretically, your IRA will increase over time to compensate for inflation and more as your investments grow. While we can never be certain that a severe depression won't occur just prior to your retirement, the odds favor your making money over time. An example is that in April, with COVID, my accounts were down $120K. It scared me just a little but, as of this week, I'm slightly ahead doe the year once again. 

$500 bucks a year for 40 years is only $20,000.  A million won't be worth what a million is worth now, but it will be worth more than $20,000 is worth now.

Probably.

If you keep your savings in dollars - whether in an IRA or under your mattress - they will lose value over time. 

If you buy real estate, gold, artwork, or other physical objects, they might gain or lose value depending on how the market changes.

What you need to do with your retirement savings is invest in stocks and bonds. Bonds are loans that pay interest. Stocks are ownership of companies, giving you a share of their profits. Either of these should increase your money faster than inflation reduces its value.

As long as you put the money into something that returns better than the rate of inflation, then it's worthwhile. There isn't really much to lose when inflation is going to eat away your savings if you do nothing.  

You want to invest it obviously, if you dont know anything about investing then just put it in an S&p 500 index fund which is already diversified and you can leave it long term

Over time, putting savings into wise equity investments beats inflation.  If the dollar isn't worth as much 30 years from now and you haven't saved any, you will wish you had.