If you are adding data labels to an existing chart you will want to navigate to Open Visible. Open the dashboard where the chart you'd like to edit is located. Click the three horizontal dots in the top right of the chart and click Edit as shown below:
This will open your chart builder.
Next, you will want to navigate to the Options tab of the chart builder
From here, you will want to toggle on Show data labels
Click Save in the top right and your data labels will be saved on your chart.
Please let us know if you have any additional questions!