I’ll try … (not for real) but as first please read and understand what is indeed written: “command not found”. While something not found within folder/directory where you are currently typing here related/correct command and move forward (while obsolete approach taken, besides wrong question asked):
cd /home/path-to-your-user-name/
cd to_the_chosen/related_script_extract_folder
Turn-on every HKS (disable related chips/cards)!
cd librem5-flash-image
ls -la
cd scripts
ls -la
uuu -lsusb
./librem5r4-flash-image
In short: adjust as necessary official or several other related links (search this Forum further for many related and quality tips … already and kindly written here for all …).