Why am I seeing a 403 Forbidden - TAX_PERIOD_NOT_ENDED error when I try to submit a VAT return?

The reason you are seeing a 403 Forbidden - TAX_PERIOD_NOT_ENDED error is because the tax period for the VAT return you are trying to submit has not yet ended. Normally it starts on the 1st of the month following your month, quarter or year end. If you think this is in error then you will need to contact HMRC (see link below) as the VAT data (open obligations) displayed in Easy MTD VAT is taken from the HMRC system.

https://www.gov.uk/government/organisations/hm-revenue-customs/contact/vat-enquiries