I installed package helm from M-x package-list-packages. I then added the minimal configuration:
(require 'helm) (helm-mode 1) in my init file. Now when I start emacs, I see the following warning message in a split window:
Warning (initialization): An error occurred while loading ‘/home/norio/.emacs.d/init.el’: Invalid function: (obsolete ansi-color-apply-sequence) To ensure normal operation, you should investigate and remove the cause of the error in your initialization file. Start Emacs with the ‘--debug-init’ option to view a complete error backtrace. I did started emacs by the command emacs --debug-init and got the following message in the *Backtrace* buffer
Debugger entered--Lisp error: (invalid-function (obsolete ansi-color-apply-sequence)) (obsolete ansi-color-apply-sequence)() byte-code("\301\302 \303\304\305\"\"\210\010\306W\203\025\0\307\310\311\304#\210\312\313!\207" [emacs-major-version with-suppressed-warnings (obsolete ansi-color-apply-sequence) defalias helm--ansi-color-apply #f(compiled-function (string) "A version of `ansi-color-apply' immune to upstream changes.\n\nSimilar to the emacs-24.5 version without support to\n`ansi-color-context' which is buggy in Emacs.\n\nModify also `ansi-color-regexp' by using own variable\n`helm--ansi-color-regexp' that matches whole STRING.\n\nThis is needed to provide compatibility for both emacs-25 and\nemacs-24.5 as emacs-25 version of `ansi-color-apply' is partially\nbroken." #<bytecode 0x14a8579>) 26 advice-add ansi-color-apply :override provide helm-lib] 5) require(helm-lib) eval-buffer(#<buffer *load*-149399> nil "/home/norio/.emacs.d/elpa/helm-core-20251009.447/helm-core.el" nil t) ; Reading at buffer position 1220 load-with-code-conversion("/home/norio/.emacs.d/elpa/helm-core-20251009.447/helm-core.el" "/home/norio/.emacs.d/elpa/helm-core-20251009.447/helm-core.el" nil t) require(helm-core) eval-buffer(#<buffer *load*-755586> nil "/home/norio/.emacs.d/elpa/helm-20251009.506/helm.el" nil t) ; Reading at buffer position 1611 load-with-code-conversion("/home/norio/.emacs.d/elpa/helm-20251009.506/helm.el" "/home/norio/.emacs.d/elpa/helm-20251009.506/helm.el" nil t) require(helm) eval-buffer(#<buffer *load*> nil "/home/norio/.emacs.d/init.el" nil t) ; Reading at buffer position 7777 load-with-code-conversion("/home/norio/.emacs.d/init.el" "/home/norio/.emacs.d/init.el" t t) load("/home/norio/.emacs.d/init" t t) #f(compiled-function () #<bytecode 0x1e0f4d>)() command-line() normal-top-level() What is (obsolete ansi-color-apply-sequence) and how to deal with it? Is this a lisp expression, or the parenthesis are there as an indication of auxiliary remark?
My emacs version is
GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.14) of 2020-03-26, modified by Debian 
apt install elpa-helminstead, because the Debian packages should be compatible with each other.