While it doesn’t happen often, there are some situations where the option to buy shares is not available.
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.
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.
The US government has placed restrictions on investments associated with particular countries or regions
On rare occasions, the US government may enact executive orders that limit the movement of money to particular companies or companies within a particular region.