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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): `25.0.13
Operating system and version (eg, Ubuntu 20.04): ubuntu (webhosting)
Apache or nginx version (eg, Apache 2.4.25): unkown (webhosting)
PHP version (eg, 7.4):8.1.
The issue you are facing:
I get the following messages:
I shall optimize the database using the following commands:
occ db:add-missing-indices
occ db:add-missing-primary-keys
occ db:add-missing-columns
occ db:convert-filecache-bigint
In addition: By executing these commands, these outstanding changes can be applied manually.
I have tried to use these commands in the nextcloud directory of my webhosting. But it is said to me that these commads are unkown.
I am simply overwhelmed by it. Is there a detailed instruction for this problem? What exactly should I do?
Is this the first time you’ve seen this error? (Y/N):
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
PASTE HERE
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE
PASTE HERE
Output errors in nextcloud.log in /var/www/ or as admin user in top right menu, filtering for errors. Use a pastebin service if necessary.
PASTE HERE