What device/IOS are you using? It seems it doesn’t know the vlan access-map command. Instead it thinks you want to create a VLAN called ‘a’ (first letter it finds) which returns an error since VLANs can only have numbers.
Here’s the output of a switch:
Switch(config)#vlan ? WORD ISL VLAN IDs 1-4094 access-log Configure VACL logging access-map Create vlan access-map or enter vlan access-map command mode accounting VLAN accounting configuration configuration vlan feature configuration mode filter Apply a VLAN Map group Create a vlan group internal internal VLAN
Above you can see it supports vlan access-maps.
Switch(config)#vlan access-map ? WORD Vlan access map tag
Switch(config)#vlan access-map TEST Switch(config-access-map)#