You are trying to change the default time zone for a country like Turkey, but you cannot uncheck the default time zone.
SAP Cloud for Customer
Reproducing the Issue
Go to the Business Configuration work center.
Open activity Countries.
Open Specify Time Zones.
The default checkbox is grayed out and cannot be unticked.
The default time zone can be changed only for localized countries. This feature is not enabled for non-localized countries like Turkey.
This is the current designed behavior of the system.
Specify Time Zones, Default Time Zones, localized countries, non-localized, , KBA , SRD-HR-PAD-GLO , Cross-country components , Product Enhancement