I got an email saying that I had a “message from a seller” the other day, but when I clicked on the button in the email to view the message, I was just taken to the login screen in AliExpress. It turns out that you can only view messages from sellers using the Desktop website (not the mobile website) and I guess also the app? However, that doesn’t mean that you can’t use your mobile to view the seller’s messages without installing the app.
To view messages from sellers on your mobile, log into the AliExpress website then click the 3 dots at the top right of your browser (I’m assume you’re using Android here, there will be another option for iPhone, I’m sure), then click the button to view the website in Desktop mode (Desktop site). This should give you access to a whole bunch of extra options on the AliExpress website, including a message center where you can view messages from sellers.