/give player minecraft:shield 1 durability:3 mending:1 basecolor:14188339 HALF_HORIZONTAL:10040115 HALF_HORIZONTAL_MIRROR:10040115 CROSS:3361970 STRIPE_CENTER:3361970 STRIPE_MIDDLE:3361970 FLOWER:16777215 name:&4C&9apt&fain_&4A&9mer&4ica's_&9Shield

.

Error: No enum constant org.bukkit.block.banner.PatternType.durability

Durability is not a value, you'll want to use minecraft:shield:3 instead

This has been fixed in 5960f55. You can grab the fix from build 417 on the CI server.