Why is Steam telling me that I've been "attempting a lot of purchases" and how do I fix it?

When trying to make a purchase, Steam keeps saying:

It looks like you've been attempting a lot of purchases in the last few hours. Please wait a while before trying again.

How do I get rid of this message to complete my purchase?

There appears to be a limit in place for how many purchases you can make for a given time. This kind of feature is usually a safeguard to prevent fraud on someones account.

One Steam Community post says that the message went away after about an hour:

About an hour, but it is super annoying. You could miss some sales if this happens to you at the wrong time!

If your purchase is specifically around the Steam Deck however, it appears Valve is actively trying to prevent scalpers (which have been an issue recently with popular product releases) from grabbing up all the pre-orders for the Steam Deck, as stated in this article.