hello im trying to push my version to test-flight every time im getting this error when im trying to validate the app on x code unable to process application at this time due to the following error: null. before i was able to build and validate with out any errors but now i dont know what is this error and how to fix please help

my app is in-app purchase with auto-renewable subscriptions after buying subscription 1- i chose to upgrade to other product i got popup asking me do you want to upgrade and will get refund i press confirm 2-after seconds i chose to upgrade again to higher subscription i got popup asking me do you want modify your subscription and subscription plan will begin and you will charged when current subscription expires my question when i upgrade for the second time why i did not get the same first popup to ask me to upgrade ? why there is no refund ? what does the second popup mean its upgrade or downgrade ? should i updated my sever directly with the new plan ?