This can happen if you haven't submitted the prior ready-to-submit batch. To fix do one of the following:
Click in and submit the prior batch first then you will be able to submit the latest batch.
If you don't require the prior batch to be submitted, then click in the batch and select the drop-down to mark it as Not Required Then you will be able to submit the latest batch, and it won't be greyed out.
See the below example: there are two Ready-to-submit batches, the top one is greyed out until the older batch is submitted or marked as not required.
