Unable to spawn entity with summon command

I've been trying to make an adventure map, but I can't summon the boss, because it keeps saying "Unable to summon object".
Here's the command I'm using:

summon minecraft:zombie ~ ~1 ~ {CustomName:"Uncle",Health:100.0f,HandItems:[{Count:1,id:diamond_sword}, {Count:1,id:shield}],UUIDLeast:-6645587150281567948L,UUIDMost:4420078606457655279L}

Solution 1:

Your problem is probably your CustomName. Since 1.13, the name must be enclosed with quotations followed by a backslash, without removing the original quotations.

An example:

summon aquid ~ ~ ~ {CustomName:"\"Zoidberg\""}

Your original command with those changes looks like this:

summon minecraft:zombie ~ ~1 ~ {CustomName:"\"Uncle\"",Health:100.0f,HandItems:[{Count:1,id:diamond_sword}, {Count:1,id:shield}],UUIDLeast:-6645587150281567948L,UUIDMost:4420078606457655279L}