IDK

explain the whole issue. What seems to be the problem? What exactly are you "writing". What plugins/ mod you are using

you have vanilla minecraft with no mods or plugins?

um you are not really helping here bud......

tell me names of the plugins.
(Goto the control panel of your server and click files------>Plugins)

share your server log

(from control panel ------> log -----> share )

i can see you were able to type in commands. I dont see a problem here

ok i feel like you're trying to use commands that are not in the list of commands of minecraft or the commands that are part of plugins.

for example :

[11:13:05] [Server thread/INFO]: gejmricek issued server command: /vanish

129
[11:13:09] [Server thread/INFO]: gejmricek issued server command: /vanish gejmricek

i dont think there is a vanish command in the plugins that youve installed thats why they are not getting executed.
But when you tried to change the time using /time set command worked because it is a default minecraft command.

Here are the list of minecraftcommands

Commands

Commands, also known as console commands and slash commands, are advanced features activated by typing certain strings of text. In the client, commands are…

minecraft.fandom.com

Like the system says type /help <pluginname> to get the list of commands supported by the plugin