You cannot really "store" lots of money in the physical sense. Money is an entitlement to benefits in the future and mostly is just numbers in some computers and typically there is always a counterpart to every transaction. This here is a case of vanishing counterparts.
Think about money in terms of goods changing ownership. Russia sold lots of natural resources and by this converted them into overseas assets. They earned the right to buy goods from their trading partners in the future. They could have done that already and brought these assets already home by increasing imports (buy gold, apples, paper clips) or by storing huge amounts of dollar bills at home. But probably both wasn't practical and anyway, nobody can live from gold or dollar bills alone. They didn't do it, otherwise the trade surplus would have reversed. So they stayed as overseas assets. And that's where they provide liquidity. They tried to diversify their foreign currency reserves away from dollar and euro also towards yen (doesn't help them now) and yuan, but only 13% of them are in yuan, and gold. The majority of their foreign assets are still in euro, dollar, sterling and yen and they are all inaccessible now.
If there is an ordinary economical crisis, having a financial cushion is great, you can support your economy by temporary deficit spending.
If however there are economic sanctions, this will not help. Trade is disrupted and your assets are temporarily (or permanently) inaccessible. Think of it as parts of the World not wanting to honor payment agreements with Russia under the present circumstances. That makes these assets pretty useless. The gold they have is also difficult to use because nowadays it's actually hard to do large-scale business with physical money like dollar bills or gold bars.
Maybe China might help out. They could increase their exports to Russia with or without compensation. Otherwise Russia's economy is basically on its own and on its own it may be worth much less than before. It may also have an unfavorable composition, not producing the stuff that Russians actually need.
Conclusion: This question stumbles over a fundamental misconception of what money is. Large amounts of money are not stored physically. They are represented by contracts with two sides to the contract, therefore they are not localized at a single position in the world but have at least two end points (one in Russia and one overseas). Now Russia was unable to move all the overseas endpoints to China where it would have been still usable. Instead a large chunk of their foreign assets had counter-parties in Europe, the US or Japan. These counter-parties have refused to honor the agreements now and this means that a huge part of the assets of Russia effectively vanished into thin air for the moment. Russia may have underestimated the unity of almost all their trading partners in this matter and the sanctions are indeed exceptional, but in the end it just means that foreign currency reserves are never completely safe. The argument about the large amount of foreign reserves of Russia and the protection it would give them was always seriously flawed.
Epilogue: The same but even one order of magnitude larger could happen to China, should for example China attack Taiwan and the rest of the world decides to conveniently ignore the $3 trillion foreign exchange reserves of China. It could make such a potential attack the most expensive attack ever.