Do not seek "Office Portable 2021." Instead, use free, legal portable suites or the web version of Microsoft 365. If you need full Office 2021 compatibility, install the real licensed version on the host PC.
A "portable" version of Office 2021 is a modified, standalone installation designed to run directly from a USB drive or an external folder without being formally installed on the host computer’s registry or Program Files. office portable 2021