When encountering errors related to petty cash not being able to post, you should always check if your tax lines exist and that the batch is balanced.
Please refer to the following article to check if the system defaults have been set up correctly and also the errors to look out for when checking the tax balance of the entire batch.
https://tphglobal.atlassian.net/l/cp/H1MaH9Bb
Another error to check for if your PC is not posting would be the vendor tax code. It should be set as T9 (out of scope) or left blank.
Vendors which are setup with the Float Tax Code of V or N will cause additional lines to be included in the Gross for VAT Tax calculations so they need to be updated to T9 or left blank.
After updating the Float Tax Code of the vendor, change the amount before reverting it back to the original amount to update the float lines.