Hello Nadezhda,
As the reported Sales Quote is still having the Approval Status as 'In Approval' it does not allow you
to delete it (As shown in error message).
This is an expected behavior.
***Please See:=>
As stated by you later you removed the 'Approval Process for Quote' from BC Scoping ,it will reflect
(i.e. Prevent approval) on New Sales Quote which will be created.
Best Regards,
Ankush.