Why can't I place a Campus district on certain tiles

Question

I have following tile setup:

        {city}
        /
     {hill A} {mountain}
        \     /
       {hill B} -- {hill C}
         /    \
  {mountain} {rainforest}

I want to put Campus district on Hill B and profit from 2 mountains and 1 rainforest but I'm simply not allowed to do so. I own Hill A, Hill B and purchased Hill C but even after that I still can't put a Campus on Hill B.

The Civilopedia entry for the Campus district does not mention any restrictions on the placement, except the adjacency bonuses.

So why am I not allowed to put a Campus on Hill B?

Update - figured it out after I read the list of reasons - Hill B had rainforest and I did not have the tech to clear it.

Answer 1

According to the wiki, the limitations are as follows

• Floodplains (unless you're Egypt).
• Strategic/Luxury Resources (Unless it was built before that strategic resource was revealed, in which case you're granted access once it is researched).
• Antiquity Sites or Shipwrecks before you've excavated them.
• Already existing Wonders/Districts
• Non-Clearable Features (e.g. If you don't have mining you can't build a district on a forest).
• Harbor Districts must be adjacent to land.
• Aerodome and Spaceport districts must be on flat land (no hills).
• Aqueducts must be one tile away from both your city center and a mountain, river, or lake tile.

It states that you can place districts on bonus resources but they will be removed.