The count may take a long time to finalize if there are many items. Each item is checked against ON HAND inventory not AVAILABLE. The web page may error out because it takes time but in the background it is progressing. In fact if you check, the adjustment is adding items. The adjustment will not change the status to CLOSED until it is completed.
The solution is to be patient and just allow it to finish. If the adjustment is interrupted you would need to delete the adjustment and de-finalize the count.
Depending on when you catch the fact that the adjustment did not finish you may have to allow negative inventory to finish the count if you have completed transactions that depleted any inventory between the adjustment not completing and the new attempt to finalize the count.