1) I am not able to install the blank template with the joomla installation manager.
Do I have to extract the zip-file first and install it manually?

Error message:
An error has occurred.
0 Call to undefined method JoomlaDatabaseMysqliMysqliDriver::query()


2) Can I install a quickstart package on an existing website or gets everything lost, when I overinstall it?

In every our Joomla templates
exist:
- Quick start - which you may install as full site and receive site like our demo
- Template - which you may install to any existed site. But template - this only set of CSS styles and setting, for create site like on demo need add and configure all components and modules
- Template documentation

Sorry, I not understand where you see this error, you install quick start or template ?

Regards
OrdaSoft team

also please check:
in our Joomla Blank template - special template for Joomla 4 and for Joomla 3.
You can't install joomla 3 template to joomla 4

Regards
OrdaSoft team

The error happens, when I want to install one of your free templates (not quickstart).
I have chosen the blan template for Joomla 4. I also get an error. The template gets installed but there is always database error as shown below.

Hello

Sorry, we can't reproduce this.
Could you give to us
full FTP and supper admin access, FTP and admin access You can send to our contact form.
ordasoft.com/contact-us-form

We need:

Link to website.

Detail description: How reproduce error. Step by step. Detail description: how we may reproduce this.

Site Super Admin:
username
password

FTP:
FTP host
FTP username
FTP password


Thanks,
OrdaSoft team.
P.S. Please do site backup 2 times per month