Heya,

the game wants you to select the "full" sim difficulty level.

Probably you selected a mode, like Sandbox, where access to research or the stock market etc. is restricted.
To change that in your current game, select the menu on top of the screen.
Select "Options" and than select as game mode "Business" (your current checkmark should be at Sandbox or Sim).

This means that the game does not restrict you any more.
You have than full control over buying shares at the stock market, doing research, restocking wares and doing negotiations for wages and ware prices.

Cheers

I borrowed this quote from here (https://gamefaqs.gamespot.com/pc/564716-theme-park/faqs/55715):

Game Mode - Sandbox, Sim, or Full (full = business)

Sandbox is the option for you if you have no idea what's going on. I'd suggest
simply turning on the tutorial and letting it guide you, but if you want to
take things a bit slow, go ahead and pick this the first time. However, you
will be limited to a small number of attractions. So I wouldn't suggest using
this mode for very long.

Sim is my prefered option. You get to research new and better attractions,
busses, and staff training. You also "get" to negotiate with your staff and
the distributors that sell you supplies for your shops. Normally during these
negotations, you're forced to concede a certain amount and pay them more money
to do the same job for you. This will raise the prices at your shops and the
amount you pay your staff monthly. However, you can actually negotiate so well
with them that you end up paying *less* money! So don't think of these
negotiations (which are non-optional and you have no control over their timing)
as something to drag you down; think of them as bonus rounds where you can get
more money to improve your park!

Full (full = business) is prefered by many people because you are able to play the stocks, buying
and selling stocks (partial ownership of a company) of other, competing theme
parks to make money. If you don't know about the stock market, the idea is to
buy stocks when the company is doing poorly and the stocks are cheap, and to
sell them when the company has reached what you think will be its peak, where
you can make the most money off your purchase. However, it's a double-edged
sword. These competing companies can also buy stock from your park. If they buy
your park out the majority of your park, you are no longer really the owner of
your park and then - at least in the PC version of the game - it's game over.
In the full version, you also have to manage your shop's inventories, buying
fries and balloons and whatnot to make sure you have those items to sell. This
is all handled automatically in the sim version of the game and as far as I can
tell, offers no real advantage. Of course, in the full version you can also
explore research and development as well as negotiations from the Sim version.

Sim Level - Easy, Medium, or Hard

This option basically determines how picky and mean-spirited your theme park's
guests will be. On harder difficulties, guests are less likely to buy items
from you, especially if you are making a large profit. This will hurt your
overall income. If you are a bit of a masochist or need to prove to yourself or
someone else that you are very good at the game, go ahead and try the harder
difficulty settings. Otherwise, you might as well go for easy. It's really just
about how you get your fun out of the game.