You can level up stadium.... but don't....

Newbies like to build lots of seats and big stadiums.... but then don't have the support (or are capped)
to fill them.... this costs you home possession advantages....

Build training facilities and medical facilities first.... they bring you boosts...

When you do decide to upgrade the stadium.... build the seats first... put butts in them and earn money...
THEN upgrade the stadium.... takes the same amount of time and money... but you can sell more tix first