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sai manojsai manoj 

in trailhead module name is Quick Start: Apex ==>Add a Method to the Class i getting an error

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The accounts are not being updated because there's a validation rule on Account object that prevented your code from running successfully. The error you got mentions it's the Account Number field. It's expecting a numberic value in this field.

Assuming you are in lightning, go and check your validation rule.

Click on Setup > Object Manager  
In the quick find, type Account
Click on the link for Account, then click on Validation Rule. 

Either make the validation rule inactive by unchecking the active checkbox or, 
go through your Accounts and make sure that all account numbers comply with the validation rule. 

Let me know if you need more clarification.