Hi Mary,
Not sure if you’ve found the answer to your question from any other sources but I’ve researched on this and I’d like to share with you the following:
Here’s a sample Posting definition that you can try for Purchasing cards:
the main account 24250 is a new AP main account you can create or setup for Purchasing card invoices instead of the default 24210

More information on Posting definition, just in case you want to reference more on it:
You can find more information regarding posting definitions here - https://docs.microsoft.com/en-us/dynamics365/finance/public-sector/posting-definitions-public-sector
You can also use the Public Sector demo data ‘PSUS’ legal entity in the see posting definitions that are setup for different types of transactions.
You cannot find the PSUS legal entity from the list of available legal entity (if coming from DAT or USMF) but you can get to it straight by changing the legal entity context of the D365FO demo environment URL:
Xxxxxxxx.dynamics.com/?cmp=PSUS&prt=ps
That will bring you to a different set of legal entities, mainly for Public sector scenarios.
Hope this helps. If you encounter any issues or challenges in testing Purchasing cards, please raise a Support ticket on it to Microsoft so that it can be investigated for you.
Kindest regards,
Neil