VHDL Type Conversion Error: cannot convert type "universal_integer" to type "MemoryArray"

When compiling my vhdl code is get a conversion error at line 28.

Error:

Error (10305): VHDL Type Conversion error at registers.vhd(28): cannot convert type " universal_integer" to type "MemoryArray"

Here is the code I am trying to compile:

library ieee;
use ieee.std_logic_1164.all;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.NUMERIC_STD.ALL;

entity registers is
    port(   load : in std_logic;                                    -- 1-bit input
            WriteAddr : in std_logic_vector(1 downto 0);    -- 2 bit write address
            Memin : in std_logic_vector(5 downto 0);    -- 6 bit input data
            clk : in std_logic;                 -- clock
            Memout : out std_logic_vector(5 downto 0)); -- 6 bit output data
end registers;

architecture arch of registers is
    type MemoryArray is array(0 to 3) of STD_LOGIC_VECTOR(5 downto 0); --array of size 4, each index holds 6 bit values
    signal Memory : MemoryArray := (others => (others => '0'));

begin
    
    process(clk)
    begin
        if rising_edge(clk) then
            if (load = '1') then
                case WriteAddr is
                    
                    when "00" =>
                        MemoryArray(0) <= Memin;
                    when "01" =>
                        MemoryArray(1) <= Memin;
                    when "10" =>
                        MemoryArray(2) <= Memin;
                    when "11" =>
                        MemoryArray(3) <= Memin;
                end case;   
            end if;
        end if;
    end process;
    
    process(WriteAddr)
    begin
                     
        case WriteAddr is
            
            when "00" =>
                Memout <= Memory(0);
                
            when "01" =>
                Memout <= Memory(1);
                
            when "10" => 
                Memout <= Memory(2);
                
            when others =>
                Memout <= Memory(3);
        end case;
        
    end process;
    
end arch;

I am essentially creating a LUT that I can get the values at each index and change the values at each index with the given Memin value.

In the first process, you have used MemoryArray, which is the type, instead of memory, which is the signal name holding the memory elements.