You have probably not done last step of installation guide for SXA installations and that is to add two SXA specific Solr cores.
SXA Cores / Indexes are defined in $path_to_your_instance\App_Config\Modules\SXA\Z.Foundation.Overrides
.
To configure and populate cores for Solr, follow these steps:
1. Create Solr Cores
Navigate to your Solr folder and copy two times “{YourSitecoreInstanceName}_master_index” folder e.g. “C:\Solr\solr721\solr-7.2.1\server\solr”:

Rename newly copied folders as “{YourSitecoreInstanceName}_sxa_master_index” and “{YourSitecoreInstanceName}_sxa_web_index”:

Navigate to both new folders and remove data subfolder and core.properties file as highlighted here:

At the end you should leave only conf subfolder under both “{YourSitecoreInstanceName}_sxa_master_index” and “{YourSitecoreInstanceName}_sxa_web_index” folders like this:

Open Solr admin UI in your browser e.g. https://solr:8983/solr/
Click on Core Admin (1) in left menu and then click on Add Core (2) button with green plus as shown below. Specify “{YourSitecoreInstanceName}_sxa_master_index” in name and instanceDir fields (3) and hit Add Core button (4). Do the same also for {YourSitecoreInstanceName}_sxa_web_index:

Open $path_to_your_instance\App_Config\Modules\SXA\Z.Foundation.Overrides
file in your text editor and change core parameters highlighted below to your newly created cores in Solr to each index accordingly (mind highlighted values for each master/web index):

2. Populate Solr Managed Schema for SXA indexes
From Launchpad, open Control Panel:

Open Populate Solr Managed Schema (1) from Indexing section, select only SXA related indexes highlighted below (2) and click Populate button (3):

This is desired outcome:

3. Rebuild SXA indexes
From Launchpad, open Control Panel:

Open Indexing Manager (1) from Indexing section, select only SXA related indexes highlighted below (2) and click Rebuild button (3):

This is the desired outcome:

That’s it!
Take a look on my detailed installation steps guide for Sitecore SXA 1.8: https://tothecore.sk/2018/11/29/sitecore-sxa-1-8-installation-guide-for-development-machine/