Hi, this is probably because your server is set up to use a HTTP and not an HTTPS connection. For security reasons it is generally discouraged to use HTTP connections in 2022 to transmit information over the internet.
Anyhow, to my knowledge, this is not a circumstance which will interrupt the setup process. To help you further, could I kindly ask you to upload a screenshot of the problem?
Thank you. So, the message you are seeing is not InvoicePlane but it’s your browser prompting you with a security warning. I suppose this is not your problem with the installation.
The select language page you uploaded does not have styling. This could mean that (1) you did not upload all the files of the application (or your browser is blocking them from loading) or (2) you are not using a release version.
Where did you download InvoicePlane from? Could you share an image of the network tab of the “developer tools” of your browser?