After installation it is not possible to login ...

[OXIAction]

/var/log/open-xchange/open-xchange.log.0 shows:

2020-09-25T10:40:59,503+0000 INFO [OXWorker-0000087] com.openexchange.login.internal.LoginPerformer.log LoginRequest(LoginPerformer.java:750)
Login:testuser IP:www.xxx.yyy.zzz AuthID:4132326a610249b1a9c5d7e16a8cc252 Agent:Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36 Clientpen-xchange-appsuite(7.10.4-8) Interface:HTTP_JSON No session created.
com.openexchange.database.schema=configdb
com.openexchange.grizzly.method=POST
com.openexchange.grizzly.queryString=<none>
com.openexchange.grizzly.remoteAddress=www.xxx.yyy .zzz
com.openexchange.grizzly.remotePort=34902
com.openexchange.grizzly.requestURI=/ajax/login
com.openexchange.grizzly.serverName=ox.domainname. de
com.openexchange.grizzly.servletPath=/ajax/login
com.openexchange.grizzly.session=76149623376657440 3.APP1
com.openexchange.grizzly.threadName=OXWorker-0000087
com.openexchange.grizzly.userAgent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36
com.openexchange.localhost.ipAddress=127.0.1.1
com.openexchange.localhost.version=7.10.4-Rev9
com.openexchange.login.authId=4132326a610249b1a9c5 d7e16a8cc252
com.openexchange.login.client=open-xchange-appsuite
com.openexchange.login.clientIp=www.xxx.yyy.zzz
com.openexchange.login.login=testuser
com.openexchange.login.userAgent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36
com.openexchange.login.version=7.10.4-8
com.openexchange.request.trackingId=1635330120-817665108

After installation was complete

The directory /etc/ox-secrets did not exist. Is this normal?

[Wolfgang Rosenauer]

This can be everything.
How do you want to authenticate? Which authentication plugin did you install (which package open-xchange-authentication-*)?

Have you created a context (as defaultcontext) and a user (testuser) already?

A directory /etc/ox-secrets does not exist at all. Please explain a bit more what you have done and want to achieve.

[OXIAction]

I used this document to install ox app suite:

https://oxpedia.org/wiki/index.php?title=AppSuite:Open-Xchange_Installation_Guide_for_Debian_10.0

open-xchange-authentication-database is installed

context and testuser are already created.

[Wolfgang Rosenauer]

Ok, What is the error message you see on the login interface and can you please open a browser web console to actually see the real server response on submission of the login?

[OXIAction]

The Error Message is "The user name or password is incorrect. (LGI-0006)"

The Json of the Response:
{"error":"The user name or password is incorrect.","error_params":["defaultcontext"],"categories":"USER_INPUT","category":1,"code":"LG I-0006","error_id":"667176127-4","error_desc":"Missing context mapping for context "defaultcontext". Login failed."}

[Wolfgang Rosenauer]

"Missing context mapping for context "defaultcontext". Login failed."

So you didn't mark one context as defaultcontext it seems.

The documentation you referred to says:

/opt/open-xchange/sbin/createcontext -A oxadminmaster -P admin_master_password -c 1 \ -u oxadmin -d "Context Admin" -g Admin -s User -p admin_password -L defaultcontext \ -e oxadmin@example.com -q 1024 --access-combination-name=groupware_standard

Note the -L defaultcontext here.
You can add it to your context via changecontext as well (as loginmapping).
You can also try to login not only with "testuser" but with "testuser@1" (if you created the context with -c 1 to see if this is the only issue.