Are you sure you have the required permissions to manage the dataset refresh settings? Lack of proper permissions can result in options being greyed out.
Otherwise, if you have switched to a live connection mode, scheduled refreshes are typically disabled, since the data is expected to be up to date with the underlying source. Live connections generally pull data directly from the source system rather than refreshing at a scheduled interval.
Assuming it is a live connection, ensure that the Azure Analysis Services model itself is being processed/refreshed as expected. If the model isn't refreshing, it could lead to the Power BI dashboard not showing updated data.