A Submit button can only be clicked when all of the section's required fields are complete and there are no invalid entries in the section.
If your IndustraForm has sections with no Submit button (in other words, the No Submit Button checkbox is selected), the last unsubmitted section in the IndustraForm with a Submit button can't be submitted until all of the required fields are completed and all of the invalid entries are resolved in this section and in all of the sections without a Submit button.
This section (the last unsubmitted section) is not necessarily the last section in the IndustraForm, but the last unsubmitted section with a Submit button.
If the Submit button is still grayed out after addressing points 1 and 2 above, this may be due to issues with the workflow in sections that don't have a Submit button. To isolation the problem, clear the No Submit Button checkbox (refer to Submit for more information) for these sections and set them to be enabled From the start (refer to Enabled for more information).