Tips depend on target shell and yours is unknown. Add a shebang.

Different shells support different features. To give effective advice, ShellCheck needs to know which shell your script is going to run on. You will get a different numbers of warnings about different things depending on your target shell.
ShellCheck normally determines your target shell from the shebang (having e.g. #!/bin/sh as the first line). The shell can also be specified from the CLI with -s, e.g. shellcheck -s sh file.
If you don't specify shebang nor -s, ShellCheck gives this message and proceeds with some default (bash).
Note that this error can not be ignored with a [[directive]]. It is not a suggestion to improve your script, but a warning that ShellCheck lacks information it needs to be helpful.

https://github.com/koalaman/shellcheck/wiki/SC2148

make-nogui.sh

@@ -1 +1,2 @@
+#!/bin/bash
 make -j4 -e NOGUI=1