A very crude guess is that you're attempting to insert a whole column in your update.
But "It doesn't work" tells us nothing.
I know Oracle's error messages are awful, but at least they give us a starting point.
Some people, when well-known sources tell them that fire will burn them, don't put their hands in the fire.
Some people, being skeptical, will put their hands in the fire, get burned, and learn not to put their hands in the fire.
And some people, believing that they know better than well-known sources, will claim it's a lie, put their hands in the fire, and continue to scream it's a lie even as their hands burn down to charred stumps.
So it should update the balance and add the deposit. This code can be run and it says trigger created. But when i try to insert into amount then i get error:
ORA-04091: table SQL_ZIZHVESYDEVONPUGGMMFKTSPB.INSÄTTNING is mutating, trigger/function may not see it ORA-06512: at "SQL_ZIZHVESYDEVONPUGGMMFKTSPB.AIFER_INSÄTTNING", line 2