This virt to cache conversions has never worked in any prior release.

[root@grant-02 ~]# lvcreate --yes  --thinpool to_cache_pool -L 500M --poolmetadatasize 100M cache_sanity
  Thin pool volume with chunk size 64.00 KiB can address at most <15.88 TiB of data.
  Logical volume "to_cache_pool" created.
[root@grant-02 ~]# lvcreate --yes  --virtualsize 100M -T cache_sanity/to_cache_pool -n virt_to_cache
  Logical volume "virt_to_cache" created.

[root@grant-02 ~]# lvcreate --yes  -L 100M  -n linear_to_pool cache_sanity
  Logical volume "linear_to_pool" created.

[root@grant-02 ~]# lvs -a -o +devices
  LV                    VG            Attr       LSize    Pool          Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices               
  linear_to_pool        cache_sanity  -wi-a-----  100.00m                                                              /dev/nvme1n1p1(150)   
  [lvol0_pmspare]       cache_sanity  ewi-------  100.00m                                                              /dev/nvme1n1p1(0)     
  to_cache_pool         cache_sanity  twi-aotz--  500.00m                      0.00   10.04                            to_cache_pool_tdata(0)
  [to_cache_pool_tdata] cache_sanity  Twi-ao----  500.00m                                                              /dev/nvme1n1p1(25)    
  [to_cache_pool_tmeta] cache_sanity  ewi-ao----  100.00m                                                              /dev/sdh1(0)          
  virt_to_cache         cache_sanity  Vwi-a-tz--  100.00m to_cache_pool        0.00                                                          

[root@grant-02 ~]# lvconvert --yes --type cache --cachepool cache_sanity/linear_to_pool cache_sanity/virt_to_cache
  WARNING: Converting cache_sanity/linear_to_pool to cache pool's data volume with metadata wiping.
  THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
  Converted cache_sanity/linear_to_pool to cache pool.
  Logical volume cache_sanity/virt_to_cache is now cached.
[root@grant-02 ~]# echo $?
0

[root@grant-02 ~]# lvs -a -o +devices
  LV                           VG            Attr       LSize    Pool                   Origin                Data%  Meta%  Move Log Cpy%Sync Convert Devices                      
  [linear_to_pool_cpool]       cache_sanity  Cwi---C---  100.00m                                              0.00   0.73            0.00             linear_to_pool_cpool_cdata(0)
  [linear_to_pool_cpool_cdata] cache_sanity  Cwi-ao----  100.00m                                                                                      /dev/nvme1n1p1(150)          
  [linear_to_pool_cpool_cmeta] cache_sanity  ewi-ao----    8.00m                                                                                      /dev/nvme1n1p1(175)          
  [lvol0_pmspare]              cache_sanity  ewi-------  100.00m                                                                                      /dev/nvme1n1p1(0)            
  to_cache_pool                cache_sanity  twi-aotz--  500.00m                                              0.00   10.04                            to_cache_pool_tdata(0)       
  [to_cache_pool_tdata]        cache_sanity  Twi-ao----  500.00m                                                                                      /dev/nvme1n1p1(25)           
  [to_cache_pool_tmeta]        cache_sanity  ewi-ao----  100.00m                                                                                      /dev/sdh1(0)                 
  virt_to_cache                cache_sanity  Cwi-a-C---  100.00m [linear_to_pool_cpool] [virt_to_cache_corig] 0.00   0.73            0.00             virt_to_cache_corig(0)       
  [virt_to_cache_corig]        cache_sanity  Vwi-aotz--  100.00m to_cache_pool                                0.00                                                                 


kernel-5.14.0-402.el9    BUILT: Thu Dec 21 10:30:03 PM CET 2023
lvm2-2.03.23-1.el9    BUILT: Tue Nov 28 02:15:42 PM CET 2023
lvm2-libs-2.03.23-1.el9    BUILT: Tue Nov 28 02:15:42 PM CET 2023
lvm2-dbusd-2.03.23-1.el9    BUILT: Tue Nov 28 02:16:01 PM CET 2023