Unfortunately, you cannot create a gamepass on the Roblox mobile app.
You will need to use the Roblox website to create a gamepass. On the website, click on the Settings tab located on the left side of the page and then select the gear icon that says “Developer Products.” On the next page, you will see a button near the top that says “Create a New Product.” Click on this to initiate the Gamepass creation process.
After making all your required selections, such as adding gamepass images and prices, be sure to click “Create!” on the top right. Your Gamepass will now be live in the Roblox store.