How to add Drupal Commerce to an existing Drupal site?
Option 1: Instructions from the community documentation
You saw the available community documentation about Drupal commerce, where it says (bold markup added here):

If you're installing Drupal Commerce on an existing Drupal site or on a new Drupal site without using Commerce Kickstart, you first need to install the latest packaged release of the following modules: ...

From my quote here, it sounds to me that it &quot;has to be possible&quot; to add Drupal commerce into an existing site.
I wonder however if it'd help to get your error resolved by taking into account this part of the community docu page:

Installing Drupal Commerce modules
When turning on the sub-modules of Commerce, you will notice that some modules are dependent on other modules. You may wish to turn them on one at a time so as to pinpoint any error messages. This is the order to satisfy dependencies:

Commerce and Commerce UI
Customer and Customer UI
Price
Line Item and Line Item UI
Order and Order UI
Checkout, Payment, Product
Product Reference
Cart and Product Pricing
Tax
Product UI
Tax UI
Payment UI


More specific: are you using all these modules? If not, could your error be caused by some dependency as mentioned in the quote above? Alse, does your error remain if you (temporarely) install/enable the modules in the list above that you don't use yet.
If you have all those modules enabled, what happens if (a) you disable them all and (b) re-enable them one at a time (as in the quote above also). It might help to pinpoint the real reason for your error message ...
Option 2: Use the Commerce Repair module
Other thoughts / approaches / attempts:

look at issue # 2312841, especially comment # 9 in it, and the suggested &quot;Commerce Repair&quot; module. Does this help to get your error resolved?

Could it be that you're experiencing the bug as described in the answer to Error When Create Drupal Commerce Product Display ? It refers to the same &quot;Commerce Repair&quot; module, a module that is &quot;looking for a New maintainer&quot;, used in over 1.000 sites. To me it appears as your best possible alternative to consider, Unless you want to start from a fresh install.