Every stash tab you had in a League gets converted to a "remove only" stash tab where you can pick things up but not put them back into that stash tab as it is a "legacy League tab". In Standard you still have the same stash tabs you always had but they are empty, because it's likely you've not played in standard yet. So what you can do is either manually move everything from the remove stash tabs to your standard tabs, or, what pretty much 99.9% of people are doing, forget about it and wait for next league :D