The installation of the PK4 TimeTracker application fails because there is a field limitation in Salesforce. 

e.g: There can only be 100 custom fields on the Activity object, click here to know more.

There are 2 custom fields related to activity in the PK4 TimeTracker application. And if there are 100 custom fields on the Activity object in your org already, then the total number of custom fields goes up to 102.

As the limitation is exceeded, you see the message as in the screenshot below and the PK4 TimeTracker app installation fails in your org.

To fix this, please check the existing custom fields on Activity in your org. Then disable the unused or duplicate fields if any and try installing the PK4 TimeTracker application.