Run CheckBackspace before expandableOrJumpable().

coc-settings.json

@@ -9,6 +9,11 @@
 			"command": "/sbin/texlab",
 			"filetypes": [ "tex","bib","plaintex","context" ]
 		},
+		"bash": {
+			"command": "bash-language-server",
+			"args": ["start"],
+			"filetypes": ["sh","bash"]
+        },
 		"golang": {
 			"command": "gopls",
 			"rootPatterns": ["go.mod"],

init.vim

@@ -30,7 +30,7 @@ filetype plugin on
 " indentation
 set tabstop=4                " tabs only count for 4 spaces
 set shiftwidth=4             " when indenting use 4 spaces
-set noexpandtab              " use tab characters
+set expandtab              " use tab characters
 set autoindent
 
 " view
@@ -63,7 +63,7 @@ inoremap <silent><expr> <TAB>
 	  \ coc#expandableOrJumpable() ? "\<C-r>=coc#snippet#next()\<CR>" :
 	  \ coc#refresh()
 inoremap <silent><expr> <S-TAB>
-	  \ coc#pum#visible() ? coc#pum#prev(1) :
+	  \ coc#pum#visible() ? coc#pum#prev(1) : "\<C-h>"
 	  \ CheckBackspace() ? "\<S-Tab>" :
 	  \ coc#expandableOrJumpable() ? "\<C-r>=coc#snippet#prev()\<CR>" :
 	  \ coc#refresh()