As people are mentioning, 3 days settlement is needed by the brokerage so that the money is considered there. If someone doesn’t have the money there, it needs to be borrowed which requires the setting up of margin account privileges.
Day trading in it’s true sense, though I think the term tends to get used loosely, means that you buy and sell within the same day..and some people really do adhere to this, but some people I think tend to apply the term to short term traders too.
Many people have a prejudice against "market timers", ..those who don’t buy for the longhaul. Restricting oneself to true daytrading can work against the individual in many ways, but so can buy & hold though people tend to be in denial of that. What slumps one day may very possibly be poised to pop up the next day.. I could never really restrict myself to daytrading though I do have a habit of not cutting losses quickly enough.
When you start trading, you are wise to understand taxation, such as the "wash rule" which always seems to get people confused, which states that you can’t claim a loss if you buy or sell the same stock again within 30 days.
I don’t know if you have your taxes done by a professional, but they can be rather messy for a beginning trader. Each trade will need to be documented for gains & losses.

Kiss your money goodbye. It is a good thing that your broker makes you wait longer because then you won’t lose all your money as quickly.

I used to day trade back in the late 90’s. I made money with my quick trades 9/10 times. The 10th time would wipe out all my previous gains and more.

Stay away from day trading. There is a reason why the greatest stock market people do not act as day traders.

Sounds like you may have limited cash in the account, i.e. insufficient funds to buy the 2nd stock prior to the 3 day clearing period for the stock you sold. If possible, place more funds into the account or apply for a margin account. If this a cash only account, such as an IRA or Roth IRA, you will not be able to buy on margin.

Settlement period. "Day trading" is for suckers. The only winner will be your broker pocketing the commissions.

Mostly all transactions settle in three business followint the day of trade. This is called the settlement day, the day the money and stock must move between both the buyer and the seller.

All broker/dealers maintain their books and records on a settlement date basis.

All purchases must be paid in full prior their sale, if you sell without making payment, your account is restricted for 90 days or until good funds are received into the account and you can not use the sale proceeds to pay for the purchase

They are telling you that you had to wait, since it seems that your money to pay for the purchase had not cleared their bank, so, like all B/D they are making you wait until settlement day.

If you want to buy and sell, do what all traders do, make sure you have sufficient money in your account to pay for all purchases prior to entering the trade, and not only on the initial purchase. Then the three days doesn’t matter. If you but three days in a row, make sure you have good funds in your account to cover all purchases.

There are many of us that buy and sell every day, in a cash account, but all our purchases are paid in full without using sale proceeds and we have never had any problems.

I’m not going into margins, since it would only confuse the issue.

Dom along with Bud68 has given some illogical advice

It sounds as though you did not have enough funds in the account to complete the original purchase.

I would confirm that you have an account with margin and that you verify with your broker that you are making trades in what is called your type-2 or margin account (this is a portion of th ewhole account – strictly a term of art in this case)

After the tech bubble burst back in 2000, most day traders lost money. Some lost all their money, and some got guns and took lives of fellow day traders, managers of the firms and themselves. After, the government? or someone felt regulation was needed. Now in order to day trade, you must show your broker that you are an experienced successful investor with a portfolio worth $XXXXXX.XX. Ask your brokerage firm what their day trading requirements are.

I agree with nobody’s answer above. Unless you have some type of restricted account, and you had the initial cash to buy the stock in your account, then you own the stock when you buy it and can sell it right away.. The settlement date issue would only keep you from buying more stock.

Now, whether day trading is good idea is a separate issue.

You need to open a margin account and at least $25,000.00 USD to daytrade by law.