While it doesn’t happen often, there are some situations where the option to buy shares is not available.
All reasons the "buy" button may not be available:
The share price drops below $1 for 60 days
If a company’s share price falls below $1 USD for 60 calendar days, you can’t buy any more shares but can still sell shares you already own. The major exchanges have listing criteria that companies must meet - if they fall below $1 for an extended period of time, there is risk that it'll be delisted.
The company has been delisted from the major exchanges
If the company no longer meets the listing criteria for the exchange, then it may be delisted. Read more here on what to do if a company you've invested in has been delisted.
The exchange has enforced a trading halt
A trading halt is a temporary suspension of trading for a particular company. Trading halts are typically enforced in anticipation of a news announcement, to correct an order imbalance, as a result of a technical glitch or due to regulatory concerns.