The nohup command is not a builtin; it can't be given a subshell as the command to run, and instead it requires an actual runnable command:

nohup bash -c 'sleep 120; echo "job done"' &

Bash is giving you a syntax error because it thinks you're trying to declare a function called "nohup", which is the only case where a bare parenthesis can appear in the second word on the command line:

nohup () { ... ; }

The error message is telling you (obscurely) that it expected to see the right parenthesis there and not the word "sleep".

The nohup command is not a keyword; it can't be given a subshell as the command to run, and instead it requires an actual runnable command:

nohup bash -c 'sleep 120; echo "job done"' &

Bash is giving you a syntax error because it thinks you're trying to declare a function called "nohup", which is the only case where a bare parenthesis can appear in the second word on the command line:

nohup () { ... ; }

The error message is telling you (obscurely) that it expected to see the right parenthesis there and not the word "sleep".