Hello,
As you can see below, I'm able to enable ISIS over the physical interface but not over the GRE interface, the error message is asking to check the log but I couldn't find anything related there, thanks!!

:~$ sudo vtysh

Hello, this is FRRouting (version 5.1-dev-MyOwnFRRVersion-g19300af).
Copyright 1996-2005 Kunihiro Ishiguro, et al.

This is a git build of frr-5.1-dev-76-g19300af
Associated branch(es):
local:master
github/frrouting/frr.git/master

router isis foo
net 49.0000.10101.0000.10101.00
metric-style wide
is-type level-2-only

routera1(config-if)# ip router isis foo
routera1(config-if)# exit
routera1(config)# exit

routera1# show isis interface
Area foo:
Interface CircId State Type Level
eth1 0x0 Config Unknown L2
routera1# conf t
routera1(config)# int gre12
routera1(config-if)# ip router isis foo
Couldn't bring up interface, please check log<<<<<<<<<<<