HI,
If there is only one work address is available then you cannot change the Main Address check-box because that would be the main work address.
On the other hand if the account you have added in the Work Addresses assignment block has multiple addresses then you can choose one of them as the main address, and in that case its possible to uncheck the main address check-box for one of them.
Suppose you have an account with multiple addresses and you want to mention both the addresses as the work address then you can change the main address check box , but for at least one address it should there. SAP is taking care of this and that is why you are not able to un-check the check-box . You can only make one address as the Main Address for the same account.
Please reward points if helpful.
Thanks
Kumar