create or replace procedure upd_balance(
p_pnr number(2),p_amount number(7,2))

is
v_balance number(7,2);
v_pnr number(2) := p_pnr;
v_amount number(7,2) := p_amount;
begin
select balance into v_balance
from player
where pnr=v_pnr;

v_balance := v_balance+v_amount;

update player
set balance=v_balance
where pnr=v_nr;
end upd_balance;
/

Well, you have not shown the error message which would save me guessing! (Use SHOW ERROR) However, I can see the following errors:

1) you cannot specify scale and precision values for parameters, i.e. should just say:

create or replace procedure upd_balance(
p_pnr number,p_amount number)...

2) Variable v_nr is not declared - you meant v_pnr!

BTW, why are you copying the parameters into variables anyway? And why not just update without selecting first? Why not just:

create or replace procedure upd_balance(
p_pnr number,p_amount number)
is
begin
update player
set balance=balance+p_amount
where pnr=p_pnr;
end upd_balance;
/

Also, beware of NULL values - you may need to code:

set balance=NVL(balance,0)+NVL(p_amount,0)

Thanks, dude!