Unable to search my existing batch number under stock adjustment for the product

This happened when the batch is already allocated in other stock operations process such as stock transfer or movement. 

If you still want to perform stock adjustment for the same batch then cancel or complete the ongoing stock operation for the batch before creating a new stock adjustment activity for same batch. 

We hope the details above were helpful for you. Please feel free to share with us if require further assistance and will be happy to take it up.