If you use mods that modify the 00_government_types file, follow the steps below:

1) Go to C:\Program Files (x86)\Steam\steamapps\workshop\content\1158310 (or your corresponding folder where the CK3 installation is located and the mods are downloaded)

2) Locate the mod you want to make compatible. To do this, you can open the various folders and read the "descriptor" file with Notepad to find the name and identify the right mod.

NOTE: I recommend downloading a program such as Notepad++ for better management and editing of these files.

3) Open the file(s) in common/governments and locate the lines containing "valid_holdings" and insert inside the parenthesis: trade_center_holding

4) You should have something like this:
valid_holdings = { castle_holding trade_center_holding }

5) This file lists the different types of government (feudal_government, theocracy_government, tribal_government, ecc...) if one of them doesn't have the line containing 'valid_holdings', add it manually just below 'primary_holding', except for mercenary_government and holy_order_government

You should have something like this:
primary_holding = castle_holding
valid_holdings = { trade_center_holding }
...