how to create a auto generated quatation number in sql

You could use an Identity column in your table.

Explanation[^]
Documentation[^]

[Edit in response to OP comment]
Primary key is irrelevant.

IDENTITY columns get automatically generated numbers. These may not be sequential as the values are unchanged after deletions (unlike using something like ROWNUMBER() where the numbers will always be regenenerated).

For example, create a sample table

create table CP2
(
	QuoteNo int IDENTITY(10001,1),
	SomeOtherData varchar(max)
)

insert into CP2 values
('item 1'),
('item 2'),
('item 3')

This will automatically generate the QuoteNo values:

10001	item 1
10002	item 2
10003	item 3

You want yours in a particular format - so format them at the point of display e.g.

SELECT 'QU-' + CAST(QuoteNo as varchar)
from CP2

QU-10001
QU-10002
QU-10003