Carefully watch the level of your mech which is displayed in the lower right of the fitting screen. There it says "VS: XX". If you do fit higher level items you also have to fight higher level opponents.
That's irrelevant at the lowest levels, you get a box saying " your matchmaking level cannot be lower than 10"
But of course it's like that so you have to learn the basics of the game against the computer, it's not hard to reach level 10 and if you are too far along the campaign mode then losing battles will put you back down, you even get a little bit of XP.
Selling your mech and ending up with nothing is a bit silly really, you could have just fought some battles and saved up the credits, that's what I did, I have a level 10, level 22 and level 40.