I think that ESS is no longer providing support for auto-complete package. The default is Company try installing it and ESS should recognize it by default. Or you might need to require it and activate it, I have it globally.
Here is my config with comments in case you need it:
(use-package company :config ;; Turn on company-mode globally: (add-hook 'after-init-hook 'global-company-mode) ;; More customization options for company: (setq company-selection-wrap-around t ;; Align annotations to the right tooltip border: company-tooltip-align-annotations t ;; Idle delay in seconds until completion starts automatically: company-idle-delay 0.45 ;; Completion will start after typing two letters: company-minimum-prefix-length 3 ;; Maximum number of candidates in the tooltip: company-tooltip-limit 10))
Alternatively you can try using Corfu.
Also, if you are using the packages from Melap you might want to update to Emacs 29, it is the most supported version right now and older version might have bugs or conflicts.
ess-r--init-error-handler: Wrong type argument: listp, "Caused by error: Symbol’s function definition is void: ess-local-process-name"when I try to start an R process on a 23.04 machine with R and emacs 27.1. I can only fix it by reverting to an older version of ess. See: (github.com/emacs-ess/ESS/issues/1140) for more details