iPhone 7 and Apple Watch Series 4 can not pair

I have purchase a eBay Refurbished

Apple Watch Series 4 40mm 44mm GPS + WiFi + Cellular Smart Watch, Good Condition

, after charging it for how many hours I turn on the Watch and started to pair it with my IPhone 7, after how many seconds a dialogue box came out stating that “ Iphone Out of Date This Apple Watch requires a newer IOS version and started unpairing the apple watch. Many times I turn on and off both device but still they same results appeared. What is the best thing to do to utilize my Apple Watch?

I hope there will be help from any of you in here.

Thank you.