Shell script throws a not found error when run from a sh file. But if entered manually the commands work

Run the script either as:

bash script.sh

or just:

./script.sh

When bash is run using the name sh, it disables most of its extensions, such as the [[ testing operator.

Since you have the #!/bin/bash shebang line, you don't need to specify the shell interpreter explicitly on the command line. Running the script as a command will use that line to find the shell.