When encountering errors related to petty cash not being able to post due to tax flags not balancing, you should always 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.
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Another error to check for if your PC is not posting would be the vendor tax code code in the Amount Advanced and Accounted lines. It should be set as T9 (blank or out of scope ) or left blankflag.
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Vendors which are setup with the Float Tax Code of V or N will cause additional the Advanced and Accounted for lines to be included in the Gross for VAT Tax calculations so they need to be updated to T9 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 linesline.