Your store can be configured to require shoppers to open an account before making a purchase. The default setting allows guests to make purchases, with an option to register for an account after they complete the checkout process.
To disable guest checkout:
On the Admin sidebar, go to Stores > Settings > Configuration.
On the left panel, expand Sales and choose Checkout.
Expand the Checkout Options section.
For a detailed description of each of these configuration settings, see Checkout Options in the Configuration Reference Guide.
If the setting is for a specific store view, choose the store view where the configuration applies.
When prompted, click OK to continue.
Set Allow Guest Checkout to No
.
If necessary, clear the Use system value checkbox to enable changes to this setting.
Click Save Config.