Hey Alice, welcome to the community!
You can categorise your ATM withdrawals with a single category at the moment.
You’ll probably want to spend your cash on several things though so you may want to vote for (like) this idea -
In the meantime, if I need to withdraw £50, I might withdraw £10 & then £40 so that I can allocate each withdrawal a different category - obviously that’s not ideal though.
Unfortunately it’s not possible to manually enter transactions in the app. The only way, that I can think of, for you to sort of keep track of how much you have left to spend, on what, after buying something cash would be to reduce your category’s Targets (an iOS feature, for now) by however much you’ve spent (again - not ideal).