Options Help - Partial allocation of shares?

I only see 50 shares available

I'm assuming that this means you only have 50 shares of ACB in your account.

Each option contract is for 100 shares. Exercising one put option contract allows you to sell 100 shares at the strike price.

You tried to exercise two put option contracts. In other words, you tried to sell 200 shares. You only have 50 shares. Therefore, "you don't have enough shares to cover the cost of this exercise".

What should you do? If you want to exercise two put option contracts, you will need to have 200 shares in your account. You should buy 150 more shares to bring the number of shares you own to 200.

P.S. Is there any valid reason why you are doing an early exercise rather than selling the put options? Won't you lose the time value when you do an early exercise?

I only see 50 shares available

This makes no sense given your comment that you don't own any shares. Moving on...

A long put gives you the right to sell 100 shares at the strike price.

You own two puts so you can sell 200 shares if you have the shares in your account.

If you do not own shares then you will be selling 200 shares short. To do so, you would have to have a margin account, the approval to sell short, and the margin available to support the trade.

Under Regulation T, the Federal Reserve Board requires all short sale accounts to have 150% of the value of the short sale at the time the sale is initiated. The 150% consists of the full value of the short sale proceeds (100%), plus an additional margin requirement of 50% of the value of the short sale.

You received the following message from Robinhood when you tried to exercise your puts:

"You don’t have enough shares to cover the cost of this exercise."

That implies that because you don't have the shares to sell, they are looking at your SMA (money available for shorting) and you don't have enough of that either. I say implies because Robinhood leaves a lot to be desired when it comes to these things.