What can be the reason that the system shows that Salesforce CLI is not installed?

 You may have two issues: The SFDX may be in the PATH environment variable, which means the path of the bin subdirectory from where you can get the sfdx executable. In that case, you can relaunch the VS code or terminal to observe any recently made changes in the Windows PATH. Also, you can quote any file or path names of a directory file containing spaces while calling SFDX in a shell. Add the SFDX location to the variable of the system path. Then, open the environment variable and choose the path present in the System Variables. But remember that the path should be present in the User Profile. After that, copy the path where the file ‘sfdx.cmd’ file is navigated to environment variables, paste the same, and then add a new value in the path. Start the Visual Studio instances and open the command line before creating a project using the command palette.