Before display categories in Tabs make sure you create and assign category to the map.
To display category tab you can follow below steps.

Step 1 Go to Add or Edit Map and scroll down to ‘Tabs Settings’ section.
Step 2 Enable ‘Display categories tabs’ option inside the Display tabs.
Category Tab

Step 3Save the map and open it in a browser.

Now you’ll see the category tab on the google maps right top corner.

Display Category Tab

Video Tutorial