I'm trying to upload plugins but every time it says "You do not have permission" and comes up with the error "The file uploaded is invalid" but I've tried different plugins and they're all XML, I don't understand.

This might sound a silly question (sorry) but you are trying to upload the .xml in the 'Plugins' section of the Admin Control Panel and not into another section of the ACP or into a post ?

Very simple process for basic plugins, please see pics:

1. Click System on the left side on the Admin Control Panel

2. Click Plugins in the list

3. Click 'Install New Plugin' on the right hand side of the plugin list page

4. Browse for and select the .xml file of the plugin you wanted to install

5. Click 'Install'

The only other immediate cause can think of is the xml you are trying to use is not a plugin its something else such as an xml for a previous version (for example a 2x or 3x 'BBCode') or suchlike.

Did you obtain the plugin from the Marketplace ? Please post the link here to it if it persists and I'll take a look.

That's what I did and it came up with the error, and yes I got it from the marketplace: 

 

Into which version of IPS 4.1 are you trying to upload this 3.x plugin? (that's the clue, by the way )

1 minute ago, Terence Mclean said:

That's what I did and it came up with the error, and yes I got it from the marketplace

Yes, this is for IP.Board 3, not IPS 4.

Is there a way I can add this plugin to my site anyway?

Are you running IPS 4.0.x or 4.1.x? If so, see if the author of the plugin has created a version compatible with those releases of the IPS Community Suite.

 

Not this version. You better request the author to make it compatible (do a new one actually) or request one on this forums https://community.invisionpower.com/forum/420-request-modifications/ or even take a look on Providers and hire someone to make it for you: https://community.invisionpower.com/third-party/providers/

That's the issue then as pointed out, its for 3x.

If the 'news' is not likely to change too often you could either make an announcement and put it there or if you have Pages (formerly IP.Content) a custom block could be put there quite easily.