This often happens in the Payment Portal when using a mobile phone, and the file is too big. Please note that the maximum size of the file should be 5MB only. Larger than the said size of the file may cause an error in uploading your attachment/s.
As an alternative solution, we suggest you open the portal link using a desktop or a laptop. Upload your proof of payment to google drive first. Then, when attaching the file to the form, get your saved proof of payment in google drive.
If the error still occurs, please do not hesitate to email us at billingandcollection@citiglobal.com.ph.