Welcome.
- You appear to be using the OTG extension described here>Serial OTG. Arduino CH340. FTDI. rkl099's Extension - #2 by Juan_Antonio and not the Serial component located in the App Inventor Connections bin.
The extension Blocks are named SerialOTG1 (what you seem to be trying to use); the App Inventor component Blocks are named Serial1.
To use the extension, your Android device needs to be OTG capable. Is your's? Do you have an OTG cable?
Regarding the extension: In the mobile Settings . Additional Settings. You must activate OTG