I also experienced this issue, although the system rebooted fine afterwards. The grub-install command continued to fail afterwards. As noted above, adding --no-nvram allows grub-install to run. I don't understand what might be lost by not updating NVRAM variables. System continues to reboot OK though.

$ sudo grub-install /dev/nvme0n1
Installing for x86_64-efi platform.
grub-install: warning: Internal error.
grub-install: error: failed to register the EFI boot entry: Operation not permitted.

$ sudo grub-install --no-nvram /dev/nvme0n1
Installing for x86_64-efi platform.
Installation finished. No error reported.
$ 

System information:
System:    Kernel: 5.4.0-54-generic x86_64 bits: 64 compiler: gcc v: 9.3.0 
           Desktop: Cinnamon 4.6.7 Distro: Linux Mint 20 Ulyana base: Ubuntu 20.04 focal 
Machine:   Type: Desktop System: Gigabyte product: X570 AORUS ELITE WIFI v: -CF 
           serial: <filter> 
           Mobo: Gigabyte model: X570 AORUS ELITE WIFI v: x.x serial: <filter> 
           UEFI: American Megatrends v: F30 date: 09/15/2020 
Battery:   Device-1: hidpp_battery_0 model: Logitech Wireless Solar Keyboard K750 
           charge: 100% status: Full 
           Device-2: hidpp_battery_1 model: Logitech Marathon Mouse/Performance Plus M705 
           charge: 55% (should be ignored) status: Discharging 
CPU:       Topology: 8-Core model: AMD Ryzen 7 3700X bits: 64 type: MT MCP arch: Zen 
           L2 cache: 4096 KiB 
           flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm 
           bogomips: 114988 
           Speed: 2195 MHz min/max: 2200/3600 MHz Core speeds (MHz): 1: 2672 2: 2180 3: 2114 
           4: 2171 5: 2195 6: 2328 7: 2002 8: 2193 9: 2194 10: 1879 11: 2192 12: 2202 
           13: 2184 14: 2172 15: 2510 16: 2018 
Graphics:  Device-1: AMD Ellesmere [Radeon RX 470/480/570/570X/580/580X/590] 
           vendor: PC Partner Limited driver: amdgpu v: kernel bus ID: 09:00.0 
           Display: x11 server: X.Org 1.20.8 driver: amdgpu,ati 
           unloaded: fbdev,modesetting,vesa resolution: 2560x1440~60Hz 
           OpenGL: 
           renderer: Radeon RX 580 Series (POLARIS10 DRM 3.35.0 5.4.0-54-generic LLVM 10.0.0) 
           v: 4.6 Mesa 20.0.8 direct render: Yes 
Audio:     Device-1: AMD Ellesmere HDMI Audio [Radeon RX 470/480 / 570/580/590] 
           vendor: PC Partner Limited driver: snd_hda_intel v: kernel bus ID: 09:00.1 
           Device-2: AMD Starship/Matisse HD Audio vendor: Gigabyte driver: snd_hda_intel 
           v: kernel bus ID: 0b:00.4 
           Sound Server: ALSA v: k5.4.0-54-generic 
Network:   Device-1: Intel Dual Band Wireless-AC 3168NGW [Stone Peak] driver: iwlwifi 
           v: kernel bus ID: 04:00.0 
           IF: wlp4s0 state: down mac: <filter> 
           Device-2: Intel I211 Gigabit Network vendor: Gigabyte driver: igb v: 5.6.0-k 
           port: f000 bus ID: 05:00.0 
           IF: enp5s0 state: up speed: 1000 Mbps duplex: full mac: <filter> 
Drives:    Local Storage: total: 4.55 TiB used: 512.77 GiB (11.0%) 
           ID-1: /dev/nvme0n1 vendor: Samsung model: SSD 970 EVO Plus 1TB size: 931.51 GiB 
           ID-2: /dev/sda vendor: Western Digital model: WD2003FZEX-00SRLA0 size: 1.82 TiB 
           temp: 48 C 
           ID-3: /dev/sdb vendor: Western Digital model: WD2003FZEX-00SRLA0 size: 1.82 TiB 
           temp: 45 C 
Partition: ID-1: / size: 62.50 GiB used: 10.58 GiB (16.9%) fs: ext4 dev: /dev/nvme0n1p3 
           ID-2: /home size: 235.23 GiB used: 51.86 GiB (22.0%) fs: ext4 dev: /dev/nvme0n1p4 
           ID-3: swap-1 size: 16.00 GiB used: 0 KiB (0.0%) fs: swap dev: /dev/nvme0n1p2 
Sensors:   System Temperatures: cpu: 40.2 C mobo: 16.8 C gpu: amdgpu temp: 35 C 
           Fan Speeds (RPM): N/A gpu: amdgpu fan: 897 
Info:      Processes: 348 Uptime: 7m Memory: 31.37 GiB used: 1.99 GiB (6.4%) Init: systemd 
           runlevel: 5 Compilers: gcc: 9.3.0 Shell: bash v: 5.0.17 inxi: 3.0.38