I believe Rabbit was trying to say nested query.
Your above syntax is an update query (update ... set ..=... where ...). Insert syntax is insert into xxx (columns) values (values).
So, if you are trying to insert use the syntax like this:
- insert into table (nr) select max(nr) from table where ....
If you are trying to update the existing value, use something like this:
- update table set nr = select max(nr) from ...
In that last case, make sure you enclose the nested query with () making sure that the where applies to the right statement (i.e. update or select).