If the Submit button is disabled, you mostly have a Javascript error on your website.
As a form comoponent Visforms relies on Javascript, which is used to display conditional field, to validate user inputs, to perform calcualations, to display multi page forms and for many other tasks. As the proper working of Javascript is so crucial to Visforms we have decided to render the submit button in a disabled state in the first place and to use Javascript to enable the button after the page was loaded successfully. Thus, if you have a Javascript error on your page, the submit button stays in the disabled state.
In order to fix this, you have to find the source of the Javascript error, which to our experience often does not come from Visforms itself, but from Javascript incompatibilities caused by other extension. Bad configurations of a field of type calculation can also cause Javascript errors.
Sometimes the problem is caused by old versions of the Javascript files, cached by your browser and can be fixed by clearing the browser cache.
You can use the browser console of your browser for debugging. After the Javascript error is fixed, the submit button will automatically be enabled.
If there are no Javascript problems indicated in your Browser Console another source of disabled buttons could be, that you have created Visforms overrides in your template in the past. Overrides, which are created based on view files of older Visforms versions may not work with newer Visforms versions. You have to remove the old overrides, then. Please check also the post on our forum
Please see also the following FAQ "Field type date not working after update to Joomla! 3.7"