Need lv 5 to start on competitive season right?

No, neither the Stadium nor the Catering need to be at least level 5 to start the season. It was just the tutorial guiding you what roughly you should be doing. After all, your ticket sales is going to be the biggest chunk of your income.

Training Center : 1. Cost a lot go get lv 2. (Is this really important? cost a lot of operating cost)

I upgraded the Training Centre when I first started to level 2 and very soon I realised the operatiing cost was not sustainable. At the end, I had to downgrade it to keep the balance sheet in check. It is a nice advantage to have but I don't think it's a priority.

Training

I think you have a strong defence. Your defenders have decent stats. already. That cannot be said about your midfielders. If they can't stop the opponents, you are going to lose a lot of goals. As you can see from the table above, defending is a very important stat. even for attackers. I suggest training your midfielders and attackers in defending.

Is OS really important or just the specific stats ?

Overall score (OS) is a quick indicator when you compare players. It gives you a quick way to figure out which is the better player without you diving into their individual stats.

Next thing i realized is i cannot do any training

This is probably because you have run out of Resource Credits in your wallet. Remember this is a blockchain game. Every action you do, upgrade stadium, train or buy players is a transaction on the blockchain. Resource Credits is the equivalent of gas fees in other blockchains. Resource Credits recover automatically. If you hover the cursor over the i next to "Resource Credits", it will tell you how long it takes to be full again.

You have been doing upgrading, training and buying players, so I am not surprised your RC has run out. I've been there before:) In order to get RC to recover faster, you need to increase your Hive Power (HP). You need to click on the dropdown next to HIVE, then choose Power Up. Assuming you have some Hive in your wallet, try powering up an extra 5 or 10. What you are actually doing is locking this amount away to pay for transactions. If you are familiar with crypto, powering up Hive means staking Hive.

And how about the formation train?

All NPCs play with 4-3-3. Players who don't set a formation play with 4-3-3. If you know that a different formation is going to have an advantage over the default formation, would you use it? I think formation training is a must if you want to try to beat better players.

Do you have RBN left?

You have signed 6 players. I remembered when I had to sign a couple pf players because of injuries and bans, my balance sheet went into the red almost instantly. Their wages were too much for us to afford. I soldiered on for a few seasons and finally worked my way back to become profitable again. If you're interested, you can find that story here.

Your goal now is to get to League 2 as you will get a HIVE reward for staying in the League. If you are in debt, HIVE earnings will be converted into RBN to pay it off.