I have installed neos via composer. After the database configuration i get this message: Package “friendsofphp/proxy-manager-lts” is not installed. How can i install this package.
Hi Christian 
Welcome to the community
What version of Neos did you install and how? What version of composer are you using?
Can you show a screenshot of that exception?
Sounds like a missing dependencies in the installation of the composer packages.
That package is a replacement for ocramius/proxy-manager ^2.1 that is compatible with PHP 7.1 and composer 1 - so I assume you running with either of the two. In that case I suggest upgrading to PHP 7.2 (or better 7.3, as even7.2 is EOL by now) if possible and composer 2. Doing a composer update should then resolve the current problem. If not, try a ./flow flow:cache:flush --force
Had the same issue after updating from 5.3. Solved it by deleting old packages and composer.lock. After a new install everything is fine. (php7.4, composer2)