Unable to pay with Garmin Pay in Connect IQ

I’m confused when you say “after filling out all the data fields”.  I have my Fenix 8 set up for Garmin Pay with a Visa credit card, and when I want to use it at a retailer, I need to enter my 4 digit pin each day (unless I take the watch off, which results in my having to enter the pin again after I put it back on).  When I finally managed to get pay for the new watchface with my watch via Connect IQ, I had already entered my pin earlier in the day, and my watch just prompted me to confirm that I wanted to use the Visa card I have set up.  That was it.  No filling out credit card details, expiration date, CVV, etc.  If Connect IQ is asking you to provide those details, then it’s not actually trying to use your watch to make the purchase.  After setting your watch up with Garmin Pay, it doesn’t actually contain those details for security reasons… only a token (certificate) that your credit card company generated to be stored on your watch to make purchases.  I’m thinking Connect IQ isn’t even attempting to use the token stored on your connected watch to make the payment.

No, no the data field I have to fill out to verify my card for using it in the connect iq app. In stores I don't need to fill in anything. I, indeed, just need to enter the 4 digit pin and then I'm able to pay. But when I try to purchase a watchface or whatsoever from the connectiq app it tells me I have to verify my card. If I click the button to do so, the page leads me to the garmin app where I have to click my bankcard and it then prompts a screen where I once again have to fill out all my data suchs as cardnumber, name, cvv and exp. date. After doing so an error message pops up saying something went wrong and I have to try again

I think I understand now.  You mentioned “the page leads me to”.  Are you using a PC with a web browser to access the Garmin store?  I’m using my iPhone with the Connect IQ app.

The error pops up on both unfortunately, if I use my android phone with the connect IQ app as well as when I use the webbrowser on my laptop as well 

I hate to say this, but it very well could be your particular bank that’s rejecting the token submission from Garmin’s CC processor, and then trying to force Garmin down another path to verify the card being used.  It could also be Garmin’s CC processor not submitting the token correctly to your bank or Gamin not submitting things correctly to the CC processor, although I’m not betting on the last two scenarios as I was able to get mine to work by unplugging my watch from my phone.  As you suggested, we’re having two very different problems.

But if my bank would be the problem I wouldn't be able to use garmin pay at a physical store, right?

Not necessarily.  Paying at a store with your watch is using NFC to pass the token to the Point-of-sale device, which then passes it to the store’s credit card processor, which then contacts your bank.  It could very well be that Garmin’s system or CC processor is causing an issue, but it could also be your bank refusing the transaction from Garmin’s CC processor for some reason, i.e. - not an authorized CC processor, etc.  Lots of steps involved.  To be honest, it’s amazing that Apple/Garmin/Samsung/etc. pay work as well as they do considering all the parties and agreements involved,

But here’s something to consider… why does Apple Pay tend to be more reliable than some of the other device-payment systems?  My guess is Apple spends a lot more money on staffing up that department to make sure all the moving parts involved are properly in place and working properly.  Garmin is a tiny company compared to Apple.

From my mobile phone - I can't buy anything from Connect IQ - just stupid message without any explanation and logs: failed :(.

My workaround was: use PC and from web browser Buy - success!