Pay to fix WordPress website

My friend, his WordPress website has some bugs that need to fix,
for example in WordPress dashboard, some functions cannot be used.
And even only can use some WordPress theme (not allow to install many WordPress theme)

I guess that it’s because of initial WordPress installation issue. (the problem started, when he moved from other blog platform to WordPress)
We’re willing to pay to fix the problem.
He has contacted some website, claims that can fix the issue, but finally still cannot fix.
So we’re asking help, from WordPress official team.
If any other source, please also help to advise to me. Thank you so much