Optimizers and all other Tigo MLPE with monitoring use serial numbers to specify which units are installed in a system. During initial installation, and anytime a units is replaced, the new serial numbers must be entered.
To do this, you must be a Installer user type on the system. System Owner user types are specifically prevented from changing serial numbers.
If you are not a Installer user type on the system, have System Owner send email to email@example.com with your Tigo user account email and granting permission to add you as in Installer to their system.
Detailed directions for Serial Number changes are here:
Physical installation details are in the Quick Start Guide as shown below: