Venezuela was having serious issues even before the price of oil took a nose dive. One example is the food shortages. They were happening well before the crisis. Centralized food markets with price controls always creates shortage. It's simple economics.

If your economic system can't efficiently feed people, more complex industries will be even harder. Not to mention building new industries.

The failures of their socialist experiment go far deeper than simply a non diversified industry. The drop in the petro industry showed immediately how fragile the massive spending schemes and nationalization was.

As Thomas Sowell said (paraphrasing), the problem with redistribution is that you can only take someone's money/property once, you can't take their ability to continually produce goods. So when that well dries up the cracks in the system become apparent.

The wikipedia article on the Venezuelan economy is a good read about the risks of centralized planned economies.

Post WW2 UK elected a socialist government which created the welfare state, comprehensive schools, state pensions and the National Health Service and nationalisation of some industries.

Why would you try socialism in those countries?

It has been tried, we call that country 'The United States' - at least, if you're over the age of 65.

Not sure what you mean. The "socialist" programs, such as Social Security and Medicare, have been massively successful. But they are the tip of the iceberg in terms of what is possible. We're talking things like Medicare for All and, at the extreme end of the spectrum, Unconditional Basic Income.