Converting a key in bytes to use with openssl - 'bad magic number' - encryption

Question

I'm trying to decrypt an AES protected XML. I have the key used to encrypt it but am struggling to apply it to the decryption process. This is part of an exercise and the key has been supplied accordingly.
The Key (Bytes) used is:
[0]: 137
[1]: 67
[2]: 248
[3]: 114
[4]: 168
[5]: 126
[6]: 229
[7]: 255
[8]: 147
[9]: 249
[10]: 177
[11]: 191
[12]: 131
[13]: 184
[14]: 242
[15]: 194
[16]: 93
[17]: 98
[18]: 209
[19]: 245
[20]: 211
[21]: 122
[22]: 198
[23]: 89
[24]: 15
[25]: 215
[26]: 137
[27]: 237
[28]: 193
[29]: 202
[30]: 126
[31]: 50
I am struggling to do this though. When using node to convert the above into a password I can use with openSSL, I get an error: bad magic number.
Node code to convert the above into a base64 string:
const bytes = [137, 67, 248, 114, 168, 126, 229, 255, 147, 249, 177, 191, 131, 184, 242, 194, 93, 98, 209, 245, 211, 122, 198, 89, 15, 215, 137, 237, 193, 202, 126, 50];
const base64Key = new Buffer.from(bytes).toString('base64'); // iUP4cqh+5f+T+bG/g7jywl1i0fXTesZZD9eJ7cHKfjI=
The openssl command I am using to try and decrypt the file (using the output above):
openssl enc -aes-256-cbc -base64 -pass pass:iUP4cqh+5f+T+bG/g7jywl1i0fXTesZZD9eKfjI= -d -p -in secretmessage.txt -out notsosecret.txt // bad magic number
I'm not sure where I am going wrong but the bad magic number leads me to believe I am not reading the provided key correctly. Questions:
Is my hypothesis that the key is being misread correct?
Is the key correct but being used incorrectly?
Does 'bad magic number' mean something else entirely?
Thanks in advance!