One of the interfaces you are trying to add does not exist, so you get an error.
As much as I wish that was true, it unfortunately isn't. Please see configuration below:
/interface vlan
add comment="L2VPN - CVID 4" interface=bo1v400 name=bo1v400v4 vlan-id=4
add comment="Customer VRF" interface=bo2v900 name=bo2v900v1204 vlan-id=1204
add comment="IRL" interface=bo2v951 name=bo2v951v1204 vlan-id=1204
/ip route vrf set [ find routing-mark=customer1 ] interfaces="$[ get [ find routing-mark=customer1] interfaces ],bo1v400v4"
bo2v900v1204 and bo2v951v1204 are already in the VRF and I would like additional interface added to the VRF, which is in this case interface bo1v400v4. If I run the following command, the command is accepted and already existing interfaces are still in the VRF:
/ip route vrf set [ find routing-mark=customer1 ] interfaces="$[ get [ find routing-mark=customer1] interfaces ]"
... but if I try to apply the following command. I get the error that follows.
/ip route vrf set [ find routing-mark=customer1 ] interfaces="$[ get [ find routing-mark=customer1 ] interfaces ],bo1v400v4"
input does not match any value of interface