I have the same issue, after reading Eli’s and james’s reply, I get the hang of it and this works for me. Open the terminal and run this command
nano ~/.bashrc
then at the end of it, add the following command
export FLYCTL_INSTALL="/home/<your.username>/.fly"
export PATH="$FLYCTL_INSTALL/bin:$PATH"
Save the file by pressing Ctrl + X, then Y, and Enter.