Use /bin/bash as the bash interpreter:
#!/bin/bash
Search the bash interpreter in the PATH environment variable with envexecutable:
#!/usr/bin/env bash
A common mistake is to try to execute Windows end-line formatted \r\n script files on UNIX/Linux systems, in this case the used script interpreter in the shebang is:
/bin/bash\r
And is obliviously not found but can be hard to figure out.