Associative arrays: error "declare: -A: invalid option"

The following seems to be a typical scenario on macOS after installing a newer Bash with Homebrew:

• /bin/bash is the old Bash, 3.2
• /usr/local/bin/bash is the new Bash that knows about associative arrays (4.0 or newer)
• type bash points to /usr/local/bin/bash and bash --version is the new one (because it resolves to /usr/local/bin/bash --version)

However, scripts with a #!/bin/bash shebang line that are run with ./script will use the old Bash (the scenario in the question). Solutions are:

• Call the script with bash script: the new Bash will be used. Disadvantage: you always have to call it like that.
• Change the shebang line to #!/usr/local/bin/bash. Disadvantage: on many systems, there is no Bash in /usr/local/bin and your script isn't portable any longer.
• Change the shebang line to #!/usr/bin/env bash. This will use the first bash in your PATH, which should be the new one. This is pretty portable; the only downside is that you don't know exactly which Bash will be executed.

See also these Q&A:

• What is the difference between "#!/usr/bin/env bash" and "#!/usr/bin/bash"?
• Why is #!/usr/bin/env bash superior to #!/bin/bash?
• Why is it better to use “#!/usr/bin/env NAME” instead of “#!/path/to/NAME” as my shebang? on Unix & Linux

Make sure the version of bash being invoked as interpreter at the top of your shell script (#!/bin/bash or whatever) is also version 4. If you're doing:

bash --version

and it's giving you v4, do a which bash to check it's location.