Tvheadend

Tried to use Autobuild.sh on a fresh Ubuntu 20.04 VM.
Installed the following packages and there Dependencies after VM Installation:

• build-essential
• debhelper
• pkg-config
• gettext
• libavahi-client-dev
• libssl-dev
• zlib1g-dev
• liburiparser-dev
• cmake
• libpcre3-dev
• libdvbcsa-dev

then rebootet and started "./Autobuild.sh -t focal-amd64" as root. The build Process failed.

I found somme noteable Log Entries:
"checking for pkg libpcre2-8 ... fail (detected <none>)"
-> libpcre2-8-0:amd64 is installed

"checking for pkg libva >=0.38.0 ... fail (detected <none>)"
-> If it is needed, why it is not listed a dependency?

"DOWNLOAD misc/staticlib/Ubuntu 20.04/x86_64/ffmpeg-6560a298f2cb58523e4d60dd9832e87248206d70.tgz / kZ54ee7ZUvsSYmb9VGSpnmoVzcAUhpBXLq8k
/usr/bin/env: ‘python’: No such file or directory
...
OpenSSL.SSL.Error: [('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')]"
-> Errors like this appear multiple Times.

Dozens of deprecated Warnings. Some examples:
"dh: warning: Compatibility levels before 9 are deprecated (level 5 in use)"

"WARNING: Building with deprecated library libavresample"

"libavfilter/avfiltergraph.c:135:5: warning: ‘resample_lavr_opts’ is deprecated [-Wdeprecated-declarations]
135 | av_freep(&(*graph)->resample_lavr_opts); | ^_~~~~~"

"libavresample/internal.h:63:10: warning: ‘AVMixCoeffType’ is deprecated [-Wdeprecated-declarations]
63 | enum AVMixCoeffType mix_coeff_type; /**< mixing coefficient type */ | ^_~~~~~~~~~~~"

I don't know what exactly leads to the failed Build Process and I hope some one can help me.
If there is an easy but maybe time intensive Way to get rid of the Deprecated Warnings, please tell me and I will do my best tp support TVHeadend.

Greetings KaeTuuN