Version 20.3R3.8 for qfx5110 is not available for downloads 

Version 20.3R3.8 was mark as End Of Life since March 29th:

03/29/2023

Reference link: https://support.juniper.net/support/eol/software/junos/

Release notes for reference: https://www.juniper.net/documentation/en_US/junos/information-products/topic-collections/release-notes/20.3/jd0e18951.html
For Juniper Network QFX5110 platforms