The Algolia Magento extension supports the Luma theme out of the box as it replaces the top.search block with its own template. Porto and other themes require a configuration change for autocomplete to work. Go to Stores > Configuration > Algolia Search > Advanced and change the setting for Search input DOM selector. This setting requires the selector of the search input element of your theme.
To display InstantSearch results, the extension uses the .columns element. If your theme doesn’t include the default .columns element, you need to change the value for Stores > Configuration > Algolia Search > Instant Search Page Results > DOM Selector to an element that works for your theme.
See the Front end (UI/UX) guide for more information on configuring your frontend to help support your custom theme.