When encountering errors related to petty cash not being able to post due to tax flags not balancing, you you will need to check if your tax lines exist and that the batch is balanced (use balance tax button).
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 tax code code in the Amount Advanced and Accounted lines. It should be set as blank or out of scope flag.
Vendors which are setup with the Float Tax Code of V or N will cause the Advanced and Accounted for lines to be included in the Gross for VAT Tax calculations so they need to be updated to out of scope 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 each float line.