If you are seeing TCPDF ERROR: Some data has already been output, can't send PDF file error that means there was probably some other error message generated BEFORE the PDF was started to be generated.

This error might be coming either from VM Invoice or any other component.
If you cannot see any other error at screen you should check the webserver logs if some error is present there.

Also, if you have been editing any files, the editor might have added some chars at the start of the source files that may cause this error. (some editors do that - in such case, reinstalling and rewriting all files should help)

Hello,

then there must have been something already outputted either by VM Invoice but possibly also by any other Joomla plugin.

You should check if
1) are there any errors in long or on screen (before the "Some data has already been output" message)
2) are there any other plugins runned before? maybe those generate something
3) also, a malformatted source code file (e.g. including some characters before <?php opening tags) maybe a reason for this; this make occur by opening a source file in a text editor which reformats line ending and resaving it; it may not be visible to user, but there may be different white space char codes that might be causing that

... of course, they may be also other possible sources of problems, but I is probably not a bug in MV Invoice selt;
if the tips given above will not help and you want us to help you resolve that at your own site, you can book a paid support