User may have to check on the In Progress Picking tab once and could ensure that there is no order still in process. Finish the picking process first so you could assign another order.
Or there might be orders already picked using the pick-by section/location or Product. And in this case, if the order is having multiple products from different sections and if the customer picks by section from a section and others are yet to pick that time also system shows the order as locked till the time the other section gets picked.
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.