Map Type won't support append method

You should be able to use the update() to handle this.  Replace your append() with update() like below...

a!update(
  data: local!options,
  index: length(local!options)+1,
  value: save!value
)

This update assumes that save!value is a map like the other values in your local!options

Thanks, I was able to do that through your example

reject(
  fn!isnull,
  a!update(
    data: ri!selectedCards,
    index: length(local!options) + 1,
    value: save!value
  )
)

Although, something that bothers me a little is that I am getting the following after some selecting and deselecting:

{
  a!map(),
  a!map(
    id: 3,
    icon: "coins",
    name: "Coin",
    desc: "A coin icon to appear"
  ),
  a!map(
    id: 2,
    icon: "building",
    name: "Condo",
    desc: "A multi-family building in which you own a unit"
  ),
  a!map(
    id: 1,
    icon: "home",
    name: "House",
    desc: "A single family home, townhouse, or duplex"
  )
}

How can I could I somehow "reject" the empty a!map()?

Thanks, I was able to do that through your example

reject(
  fn!isnull,
  a!update(
    data: ri!selectedCards,
    index: length(local!options) + 1,
    value: save!value
  )
)

Although, something that bothers me a little is that I am getting the following after some selecting and deselecting:

{
  a!map(),
  a!map(
    id: 3,
    icon: "coins",
    name: "Coin",
    desc: "A coin icon to appear"
  ),
  a!map(
    id: 2,
    icon: "building",
    name: "Condo",
    desc: "A multi-family building in which you own a unit"
  ),
  a!map(
    id: 1,
    icon: "home",
    name: "House",
    desc: "A single family home, townhouse, or duplex"
  )
}

How can I could I somehow "reject" the empty a!map()?

a!localVariables(
  local!list: {
    a!map(),
    a!map(
      id: 3,
      icon: "coins",
      name: "Coin",
      desc: "A coin icon to appear"
    ),
    a!map(
      id: 2,
      icon: "building",
      name: "Condo",
      desc: "A multi-family building in which you own a unit"
    ),
    a!map(
      id: 1,
      icon: "home",
      name: "House",
      desc: "A single family home, townhouse, or duplex"
    )
  },
  remove(local!list, where(local!list.id = null(type!Integer)))
)

Thanks. That was exactly what I needed. There goes my complete answer:

link: a!dynamicLink(
  value: fv!item,
  saveInto: a!save(
    ri!selectedCards,
    if(
      ri!allowMultiple,
      if(
        and(a!isNotNullOrEmpty(save!value.id),
        a!isNotNullOrEmpty(ri!selectedCards),
        contains(
          tointeger(ri!selectedCards.id),
          tointeger(save!value.id)
        )),
        difference(
          ri!selectedCards,
          { save!value }
        ),
        
        a!localVariables(
          local!list: reject(
            fn!isnull,
            a!update(
              data: ri!selectedCards,
              index: length(local!options) + 1,
              value: save!value
            )
          ),
          remove(
            local!list,
            where(local!list.id = null(type!Integer))
          )
        )
        
      ),
      save!value
    )
  )
),

It's working as expected now.

Thank you both