i3-gaps-4.20.1-1.fc37 fails to build because "sources" file does list the source archive for 4.20.1:

ENTER ['do_with_status'](['bash', '--login', '-c', '/usr/bin/rpmbuild -bs --target x86_64 --nodeps /builddir/build/SPECS/i3-gaps.spec'], chrootPath='/var/lib/mock/fedorarebuild-x86_64-X5NGfxcZH_/root'env={'TERM': 'vt100', 'SHELL': '/bin/bash', 'HOME': '/builddir', 'HOSTNAME': 'mock', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'PROMPT_COMMAND': 'printf "\\033]0;<mock-chroot>\\007"', 'PS1': '<mock-chroot> \\s-\\v\\$ ', 'LANG': 'C.UTF-8'}shell=Falselogger=<mockbuild.trace_decorator.getLog object at 0x7fbe08730370>timeout=0uid=11527gid=900user='mockbuild'nspawn_args=['--capability=cap_ipc_lock', '--bind=/tmp/mock-resolv.70er6kle:/etc/resolv.conf', '--bind=/dev/btrfs-control', '--bind=/dev/loop-control', '--bind=/dev/loop0', '--bind=/dev/loop1', '--bind=/dev/loop2', '--bind=/dev/loop3', '--bind=/dev/loop4', '--bind=/dev/loop5', '--bind=/dev/loop6', '--bind=/dev/loop7', '--bind=/dev/loop8', '--bind=/dev/loop9', '--bind=/dev/loop10', '--bind=/dev/loop11']unshare_net=TrueprintOutput=False)
Using nspawn with args ['--capability=cap_ipc_lock', '--bind=/tmp/mock-resolv.70er6kle:/etc/resolv.conf', '--bind=/dev/btrfs-control', '--bind=/dev/loop-control', '--bind=/dev/loop0', '--bind=/dev/loop1', '--bind=/dev/loop2', '--bind=/dev/loop3', '--bind=/dev/loop4', '--bind=/dev/loop5', '--bind=/dev/loop6', '--bind=/dev/loop7', '--bind=/dev/loop8', '--bind=/dev/loop9', '--bind=/dev/loop10', '--bind=/dev/loop11']
Executing command: ['/usr/bin/systemd-nspawn', '-q', '-M', '04b1119647574576b8046522ff223b01', '-D', '/var/lib/mock/fedorarebuild-x86_64-X5NGfxcZH_/root', '-a', '-u', 'mockbuild', '--capability=cap_ipc_lock', '--bind=/tmp/mock-resolv.70er6kle:/etc/resolv.conf', '--bind=/dev/btrfs-control', '--bind=/dev/loop-control', '--bind=/dev/loop0', '--bind=/dev/loop1', '--bind=/dev/loop2', '--bind=/dev/loop3', '--bind=/dev/loop4', '--bind=/dev/loop5', '--bind=/dev/loop6', '--bind=/dev/loop7', '--bind=/dev/loop8', '--bind=/dev/loop9', '--bind=/dev/loop10', '--bind=/dev/loop11', '--console=pipe', '--setenv=TERM=vt100', '--setenv=SHELL=/bin/bash', '--setenv=HOME=/builddir', '--setenv=HOSTNAME=mock', '--setenv=PATH=/usr/bin:/bin:/usr/sbin:/sbin', '--setenv=PROMPT_COMMAND=printf "\\033]0;<mock-chroot>\\007"', '--setenv=PS1=<mock-chroot> \\s-\\v\\$ ', '--setenv=LANG=C.UTF-8', '--resolv-conf=off', 'bash', '--login', '-c', '/usr/bin/rpmbuild -bs --target x86_64 --nodeps /builddir/build/SPECS/i3-gaps.spec'] with env {'TERM': 'vt100', 'SHELL': '/bin/bash', 'HOME': '/builddir', 'HOSTNAME': 'mock', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'PROMPT_COMMAND': 'printf "\\033]0;<mock-chroot>\\007"', 'PS1': '<mock-chroot> \\s-\\v\\$ ', 'LANG': 'C.UTF-8', 'SYSTEMD_NSPAWN_TMPFS_TMP': '0', 'SYSTEMD_SECCOMP': '0'} and shell False
sh: line 1: perl: command not found
error: Bad source: /builddir/build/SOURCES/i3-4.20.1.tar.xz: No such file or directory

You probably forgot to do "fedpkg new-sources i3-4.20.1.tar.xz" when updating the spec file.