Funds that are only being tracked externally cannot be rebalanced. Portfolio rebalancing means making changes in your existing funds/asset allocation so that it matches the recommended funds/asset allocation as per your goal plan.
The portfolio we recommend (funds & asset allocation) depends on your goal plan that is determined by the type of goal you have selected, the duration (remaining duration) of the goal, and the risk level of the goal. That is why we can only recommend rebalancing funds that are inside a goal.
If you would like these external funds to be rebalanced, you can click on the “Move funds to goals” option on the external tracking page > select funds > select the goal in which you want to move them. Once moved, you can check if your funds are eligible for rebalancing. You can read in detail about how to track external funds and move them to goals, here.
We also have YouTube videos available for all the common FAQs on SIP and Portfolio Rebalancing. This will hopefully make it simple to understand the entire process and help clarify any doubts that you may have.